20200229 - v2.5.0

• purify and refactor, remove useless codes.
• new command modifier g:asyncrun_program.
• the -program=xx supports msys, mingw32, mingw64 and cygwin:

Try this on windows:

:let g:asyncrun_msys = 'd:/msys32'
:AsyncRun -program=msys ls -l && echo $(MSYS_CWD)
:AsyncRun -mode=term -program=mingw32 ls -l && echo $(MSYS_ROOT)

The -program=xx can be totally customized now:

:let g:asyncrun_program.display = { opts -> 'echo ' . opts.cmd }
:AsyncRun -program=display ls -la

The origin command ls -la will be modified to echo ls -la in the display modifier.

The msys, mingw32 and wsl are implemented as command modifiers.