This adds initial support for the `go.mod` file.  It adds the
followings:

* Syntax highligthing. We highlight keywords, strings, operator and
semver version. It works pretty well for now.
* Auto fmt on save. Command `:GoModFmt` or `Plug(go-mod-fmt)` for custom
  mappings

Before we fully support the semantics of go.mod, I think this initially
will be helpful because I discovered that `go.mod` is read and edited a
lot. So going forward, this will make it easier experimenting with Go
modules.

related: #1906

* Fix auto toggle command
* Add guard agains Go versions lower than 1.11
* Add documentation
* Fixed setting correct filetype
* Fixed parsing errors
* Changed variable names to be consistent