Check for updated template during "template use" and prompt for option

[rawkode]

It would be cool if boilr prompted/informed people that the template they're using has been updated and if they wish to update it.

[tmrts]

After integrating version control #11 to boilr, I'm thinking of using the command boilr template update for updating templates.

Not sure about regularly making requests though, since it'll be slow

[rawkode]

With the recent release of 0.3.0 and the switch to src-d/go.git, is this something I can work on? 😄

[tmrts]

@rawkode absolutely 😄

Now that we are using git for plumbing, a lot of possibilities has opened up (changelog via git log, storing metadata in .git, git hooks/subtrees etc.)

Also see #39