Added support for data subfolders

[LaurentGoderre]

This line

https://github.com/assemble/generator-assemble/blob/master/app/templates/Gruntfile.js#L72

could become

data: '<%= config.src %>/data/**/*.{json,yml}',

This would help accommodate the i18n approach out of the box

[jonschlinkert]

noted, I'm working on a new version, I'll see if I can work this into it

[LaurentGoderre]

Also, since the generator uses bower, the bower_components folder should be added to gitignore