Add cfg/config.c:cfg_set_shell()

Sets shell invocation command.

To keep this code in one place.
Grouop optional fields in the bottom of status_t
Fix closing brace styling in status.h
State in code that status init requires cfg

So initialization of cfg must precede initialization of curr_stats.
Introduce shell type to curr_stats

Specifies type of shell.
On Windows check for cmd shell only once

When it's set instead of checking it every time it is used.
Fix extending filetypes missing macro on Win&cmd

Fixes #41.

Thanks to Daniel Polanco, a.k.a. dlpolanco.
Fix work with filenames with specchars on Win&cmd

Resolves #41.

Thanks to Daniel Polanco, a.k.a. dlpolanco.
Fix app GUI subsys on Windows for path w/ spaces
Prettify vifm.c:run_converter()

Mainly its Windows version.
Merge branch 'win-fix-cmd-escaping'