Document g:emmet_install_only_plug

[noahbrenner]

g:emmet_install_only_plug is a wonderful feature! It would be great to have it documented in both doc/emmet.txt and README.md (I only found the option by searching through the source code).

If you'd prefer to have help with this, I'm up for submitting a PR.

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Here's some more detail for anyone running across this issue who might not be familiar with the option:

The following will cause emmet.vim to create <plug>(...) mappings, but not <c-y>... mappings:

let g:emmet_install_only_plug = 1

This allows you to define your own mappings without requiring all potential mappings to be created. I've found this to be handy when I want to:

1. Create mappings for only some of Emmet's functionality, and
2. prevent the remaining default <c-y>... mappings from being created automatically, in order to
3. avoid a delay when using vim's built-in ctrl-y and i_ctrl-y bindings (vim otherwise has to wait, to make sure you're not about to enter the second part of a longer mapping).

A simple example:

let g:emmet_install_only_plug = 1
nmap <Leader>, <plug>(emmet-expand-abbr)
imap <Leader>, <plug>(emmet-expand-abbr)