Improved support for generator options

[snowman-repos]

Made these changes because I wanted to use stylus but was surprised to see that even when you select stylus it still uses sass as well. This update ensures that all generator options produce the app files as expected.

• If you select sass then main.scss is added and not main.css
• If you select stylus then main.style is added and not main.css or main.scss
• If you select sass and bootstrap then the sass bootstrap will be included
• If you select stylus and bootstrap then the stylus bootstrap will be included
• If you select coffeescript then only the coffeescript files, and not JS files, will be added, also coffeescript gulpfile
• If you select jade then index.jade will be added and not index.html
• jQuery is optional but will be included if you select bootstrap as jQuery is a dependency of bootstrap

[randylien]

Thanks for your great efforts to improve it!