check for existing f mapping, but not fx mapping

mapcheck() is too broad and doesn't cover use cases such as described
in issue #25
<expr> mapping instead of feedkeys() hack

- fixes #21: temporary sneak-reset f/F/t/T mappings break macros
- fixes #25: ; and , broken if fx (but not f) is mapped
Merge branch 'master' into test
Add test for case where fx (but not f) has a custom map / #25