TODO

The following is greatly inspired by python-crontab (https://pypi.python.org/pypi/python-crontab), Wikipedia (https://en.wikipedia.org/wiki/Cron#CRON_expression).

Field Name    Mandatory Allowed Values  Special Characters      Extra Values
Minutes       Yes       0-59            * / , -                 < >
Hours         Yes 	0-23            * / , -         	< >
Day of month  Yes 	1-31            * / , -                 < >
Month         Yes 	1-12 or JAN-DEC * / , -         	< >
Day of week   Yes 	0-6 or SUN-SAT 	* / , -         	< >
Year          No        2015-2100       * , -

Extra Values are < for minimum value, such as 0 for minutes or 1 for
months. And > for maximum value, such as 23 for hours or 12 for
months.

Supported special cases allow crontab lines to not use fields. These
are the supported aliases which are not available in SystemV mode:
Case            Meaning
@reboot         Every boot
@hourly         0 * * * *
@daily          0 0 * * *
@weekly         0 0 * * 0
@monthly        0 0 1 * *
@yearly         0 0 1 1 *
@annually       0 0 1 1 *
@midnight       0 0 * * *

Comma ( , ) Commas are used to separate items of a list. For example,
    using "MON,WED,FRI" in the 5th field (day of week) means Mondays,
    Wednesdays and Fridays.

Hyphen ( - ) Hyphens define ranges. For example, 2000-2010 indicates
    every year between 2000 and 2010 AD, inclusive.

Percent ( % ) Percent-signs (%) in the command, unless escaped with
    backslash (\), are changed into newline characters, and all data
    after the first % are sent to the command as standard input.

L 'L' stands for "last". When used in the day-of-week field, it allows
    you to specify constructs such as "the last Friday" ("5L") of a
    given month. In the day-of-month field, it specifies the last day
    of the month.

W The 'W' character is allowed for the day-of-month field. This
    character is used to specify the weekday (Monday-Friday) nearest
    the given day. As an example, if you were to specify "15W" as the
    value for the day-of-month field, the meaning is: "the nearest
    weekday to the 15th of the month." So, if the 15th is a Saturday,
    the trigger fires on Friday the 14th. If the 15th is a Sunday, the
    trigger fires on Monday the 16th. If the 15th is a Tuesday, then
    it fires on Tuesday the 15th. However, if you specify "1W" as the
    value for day-of-month, and the 1st is a Saturday, the trigger
    fires on Monday the 3rd, as it does not 'jump' over the boundary
    of a month's days. The 'W' character can be specified only when
    the day-of-month is a single day, not a range or list of days.

Hash ( # ) '#' is allowed for the day-of-week field, and must be
    followed by a number between one and five. It allows you to
    specify constructs such as "the second Friday" of a given
    month.[13]

Question mark ( ? ) In some implementations, used instead of '*' for
    leaving either day-of-month or day-of-week blank. Other cron
    implementations substitute "?" with the start-up time of the cron
    daemon, so that ? ? * * * * would be updated to 25 8 * * * * if
    cron started-up on 8:25am, and would run at this time every day
    until restarted again.[14]
    (WON'T BE IMPLEMENTED)

Slash ( / ) In vixie-cron, slashes can be combined with ranges to
    specify step values.[6] For example, */5 in the minutes field
    indicates every 5 minutes (see note). It is shorthand for the more
    verbose POSIX form 5,10,15,20,25,30,35,40,45,50,55,00. POSIX does
    not define a use for slashes; its rationale (commenting on a BSD
    extension) notes that the definition is based on System V format
    but does not exclude the possibility of extensions.[5]