vim-plugins: honor the plugin manager re filetype and syntax

Currently, all the filetype and syntax are enabled *after* all the plugins has
already been loaded. Whilst this is the case for Pathogen, it's not
recommended when using VAM.

This commit applies the recommendation for:
- VAM[0]: The filetype and syntax are enabled *before* the plugins are loaded.
- Pathogen[1]: The filetype and syntax are enabled *after* the plugins are loaded.
- Plug[2]: The filetype and syntax are automatically enabled.

[0]: https://github.com/MarcWeber/vim-addon-manager/tree/d9e865f3c2de5d9b7eabbc976f606cf1b89e29ea#recommended-setup
[1]: https://github.com/tpope/vim-pathogen/blob/a553410f1bdb9000fbc764069f3a5ec3454a02bc/README.markdown#runtime-path-manipulation
[2]: https://github.com/junegunn/vim-plug/blob/2f5f74e5e67f657e9fdac54891a76721bcd3ead3/README.md#usage