Add ability to specify default directory and omit .svg extension

[n-older]

It would be nice to be able to specify a default subdirectory like images that automatically gets prepended to the path we pass into svg while also omitting the file extension as it seems very redundant given the nature of the method.

Example:

A file existing at src/images/icons/check.svg would allow us to simply write erb <%= svg 'icons/check' %> instead of erb <%= svg 'images/icons/check.svg' %>

[ayushn21]

Great idea ... it'd definitely reduce the noise .... the change would have to be backward compatible though so no existing sites break.

I don't know when I'll get time to work on this so if you'd like to create a PR for it then I'm happy to accept this change, otherwise I'll work on it when I get some time.

Thanks :)

[n-older]

No worries! Just something I thought I'd mention since I'm used to writing it the shortened way from years of using Middleman.