Remember mappings until <cr> is hit

A certain branch tried to execute buffer-local mappings after calling
:enew. Therefore the mappings didn't exist anymore.

References #30.
Unlet s:marked after execution

This lead to nasty bugs because Startify would take the wrong branch to
open the new buffers.

References #30.
Prevent empty buffers

References #30.
Merge branch 'buffer-improvements'

Closes #30.