impl: add vim-go-impl

[fatih]

This adds the plugin https://github.com/rhysd/vim-go-impl with the
following changes/improvements:

• :GoImpl can be now called without any arguments, in this case it
  assumes the identifier under the cursor to be the receiver type
• :GoImpl io.Writer is a valid usage now, in this case it assumes the
  identifier under the cursor is the receiver type
• Removed the restriction for completion that there needs to be at least
  three words. This is neede because :GoImpl io.Writer is now a valid
  expression.
• We use now go doc for the auto completion. This allows for example
  the following usage as well :GoImpl json., in this case you can now
  complete to json.Unmarshaller, whereas godoc is not capable of doing
  it.
• There is a newline prependend before we put the generated method stubs
• Refactored to use internal functions of vim-go
• Moved to autoload and load it lazily
• Added docs to docs/vim-go.txt

Closes #122

Demo:

/cc @rhysd

[rhysd]

LGTM 👍 Thank you for the integration ❤️

[fatih]

@rhysd this is now merged. Would you like to add a note to https://github.com/rhysd/vim-go-impl to forward people to vim-go? Thanks to you too for the initial plugin!

[rhysd]

@fatih I added a note to an introduction in README.

[fatih]

Thanks @rhysd 👍