Parse all dates correctly

[lucbpz]

This PR solves #3
Adds some functions for date parsing, following these rules:

If there is no date or year, should return an empty string.
If there is no month, pick 12
If there is no day, pick last day of month

[joshuatz]

Looks good! There is just one tiny revision I'd like.

[joshuatz]

@lucbpz Thanks for the contribution! Merged into master, and I'll bump the version number and push a new release to the Chrome web store sometime today.

[mullwaden]

Hi,
I can see why you made this fix, but it can be problematic at times. A profile might, on a work experience for example, have a date range that is "2007 - oct 2007". The above code will interpret that as "2007-12-31 - 2007-10-31".

Start ends up bigger than end. I would suggest the fix is the following:

• for end dates continue as is done currently ("year-12-31")
• for start dates chose instead "year-01-01"

Maybe this will break a lot of things for people... Otherwise I am happy to make a PR.