Remove non-existent dirs from history on :history
Optimized speed of checking directory existence

Important on displaying directory history.
Better interface for view history navigation
Skip nonexistent directories on Ctrl-O/Ctrl-I

This is not only convenient (at least for me), but also more consistent
with content of the :history menu for a view.
Add basic unit tests for is_dir()
Merge branch 'dirhist-improvements'

Optimized speed of checking existence of a directory (important on
displaying directory history).

Skip nonexistent directories on Ctrl-O/Ctrl-I.
Add =pattern special form of user-defined command

Sets local filter to the given value (alike "/pattern" and "filter value").

Thanks to filterfalse.
Extract commands.c:try_handle_ext_command()

Handles most of command handling variants.
Small code clean up/improvement in commands.c
Merge branch 'add-local-filter-udf'

Add =pattern special form of user-defined command that sets local filter
to the given value (alike "/pattern" and "filter value").

Thanks to filterfalse.