Allow function values for FREEZER_SKIP_EXISTING
===============================================
This enables more granular control over what files to skip.
Fixes #86.
(Implements the idea in https://github.com/Frozen-Flask/Frozen-Flask/issues/86#issuecomment-334286530.)
Static libraries are already built so I'm not sure what this accomplishes.
```console
$ brew ls ffmpeg | grep '.a$'
/usr/local/Cellar/ffmpeg/3.3.4/lib/libavcodec.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libavdevice.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libavfilter.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libavformat.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libavresample.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libavutil.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libpostproc.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libswresample.a
/usr/local/Cellar/ffmpeg/3.3.4/lib/libswscale.a
```