Add -t flag to pass in a path to the CSS template

Hey, Thanks again for this awesome app.  Here's another feature I needed.

I added the [--template <absolute path to template>] and
[--t <absolute path to template>] flags to `fontcustom compile` to allow
for overriding of the default CSS template file.

I'm not very well versed in Ruby, so please feel free to comment and/or modify, but I think it's pretty useful to allow users to override the default `fontcustom.css` template so that it can more specifically meet their needs.

Thanks again!