Enable support for months and years

[amotl]

trafficstars

Dear Will,

first things first: Thanks a stack for conceiving this excellent package.

Attaching to #7 and #13, while recognizing your arguments, I still believe it would be reasonable to assume "30 days for a month" and "365 days for a year" as the basis for calculations with those units. I agree that this approximation should be documented well.

Based on what has been done with #10 by @duramato, this patch activates the corresponding lines.

With kind regards, Andreas.

[amotl]

Hi again,

we also submitted this patch to https://github.com/onegreyonewhite/pytimeparse2/pull/1, where @onegreyonewhite seems to have taken over maintenance of this fine package.

With kind regards, Andreas.