New options: vimrplugin_compldir and vimrplugin_tmpdir
- Move files created by R for omni completion and syntax highlight of R
  functions to $AppData in Windows and to ~/.cache/Vim-R-plugin in Linux, and
  let users change these directories paths.

- Fix bug: omnicompletion was not working when the library has their omnls_
  and fun_ files written for the first time.

- Avoid E341 (Internal error: lalloc(0, )) in a more efficient way.