Add support for absolute paths with the new option "base"

First of all, thank you to have inserted the new option ( https://github.com/jonkemp/gulp-useref/issues/141#issuecomment-154697616 ).

The issue is simple, this new option only supports paths relative to the script location.
Useref should be able to understand when the _base_ path is absolute or not, using the native path.isAbsolute(). That's it :)

PS: The absolute path support is already implemented for the _searchPath_ option.