Change to save version and date to global variables, instead of comments

[deathcap]

imho, it is more convenient to have the git version and dates available in global variables, rather than comments. Users can then open the browser console to print the values of these variables, or other code can be written to look at these variables and display them wherever appropriate (about dialogs, diagnostic reports, etc.). The values of course can still be visible by viewing the source.

This PR uses GIT_VERSION and CREATED_AT, but I'm not attached to these names (better alternatives to avoid possible naming clashes? something on a _browserify_commit_sha object? maybe it should be an option? idk)

(As a side-effect, this change also allows browserify-commit-sha to easily work with coffeeify, or other transforms from languages that have the same variable assignment and string literal syntax, but a comment syntax differing from JavaScript. If, for some reason, they run after this transform)

[kumavis]

I like this. I suggest:

• use global instead of window
• more unique variable names, these are globals after all. Maybe even give them 'the dunder': __BROWSERIFY_META_DATA__. I'd be happy even renaming the module. Room for all kinds of feature bloat!

I think we can assume that we'll get javascript, the user just has to think about the ordering of the transforms, which they should think about anyways if they are doing any transpiling.

[kumavis]

Added you as contributor on GH and as owner on npm :smile_cat: go nuts.

[deathcap]

Cool thanks, I'll treat it well