Add support for checking if the year is correct

[Shabirmean]

• Currently it only checks if the file has the license header and does not validate the year on the license
• Would be nice to check if the year on the header matches the one passed in as the argument or the default one (current year)

[willnorris]

Hey @Shabirmean, I'd be curious to hear your reaction to my comment on #60 about leaving off the current year entirely, as some have done.

[Shabirmean]

I see! The year necessarily is not important as per guidelines as long as the file has the year in which it was first create. Cool!