Previously using `:GoDebugStart` without `dlv` installed would give the error:

	E117: Unknown function: go#debug#Start

Which is not very clear. With this it's:

	vim-go: could not find 'dlv'. Run :GoInstallBinaries to fix it

Which is a lot better :-)

Otherwise they will be lost when the using `:colorscheme` and some
other commands.

Inheriting it from the colour scheme is problematic, since it can be anything.

In my case (the default colour scheme), it's black. Black on dark blue is hard
to read :-)

I think this makes more sense? No need for a long path for e.g. `a.go`.

Stuff like e.g. `/usr/lib/go/src/runtime/proc.go` is still shown as the full
path.

To allow folk to configure their own window layout, or even hide certain
windows.

In case folk already have something running on port 8181 or something

Some small improvements for GoDebug

Previously it would error out with some undefined error.

Display error for Vim 7.4

This sets up the structure for the documentation for the new debug mode;
it's not 100% complete yet. I also added a bunch of TODOs There are a
bunch of TODOs sprinkled around for stuff I noticed that behaved
unexpected/buggy.

I added it as a new section instead of putting it in the regular
commands/etc. sections. This seemed to make the most sense to me
personally: this way there is an obvious location for a "getting
started" section, and we can keep the docs for :GoDebugStart as a short
and to the point reference doc.It also avoids having to add "This
command/mapping is only available after using :GoDebug" on all the
commands.

I can move stuff around if you want.