Fix many subtle issues regarding on-demand loading etc.

- On-demand loading
    - Fix loading of unwanted files (e.g. colors/*.vim, syntax/*.vim, etc.)
- Filetyp-based on-demand loading
    - Load `after/ftdetect` as well
    - Make sure indent files are loaded by invoking
      `doautocmd filetypeindent FileType`
- Ensure plugin loaded when it was added after Vim started
- Do not reload $MYVIMRC after installtion/update
    - Instead simply call plug#end()