Make possible to choose the extension strategy

The placeholder selector strategy doesn't work very well in non modern
browsers since it may create huge CSS selectors chain and the browser
will not parse they.

So, for this case is better to use the mixin strategy.
Add the mixin option to the CLI
Update the README with the mixin option
Extract a extension_prefix helper
Define mixin for everything when using the --mixin option

This will make easier to work with media queries
Whitespaces
Update the usage with the --mixin option
Fix grammar
:scissors:
Merge pull request #9 from lucasmazza/rm-mixin-option

Make possible to choose the extension strategy