Feature request: exclude breakpoints with use_external

[gwenvanhee]

It would be nice to be able to exclude certain breakpoints when exporting to an external file.

Example use:
In [src].css I want to combine media queries for mobile landscape (eg. up to 40em), but don't want them to be exported in a separate file. Whereas [src].responsive.css would only contain media queries for tablet and desktop (40em and up).

The why here is Mobile First, where tablet & desktop styling is being served as a separate file. The plugin already does a great job at facilitating this workflow, but I'd hate to give up styling mobile landscape...

[toptalo]

+1