Fix add attachment mapping tag in vimdoc

Patch by filterfalse.
Add K mapping to Vim plugin

Provides quick navigation to documentation, e.g. from vifmrc.

Patch by filterfalse.
Allow for one-shot when vifm docs aren't in 'rtp'

Try to find it and setup for one :help command.
Several renames for consistency in the plugin
Merge branch 'add-plugin-k-mapping'

Add K mapping to Vim plugin (quick navigation to documentation, e.g.
from vifmrc).

Patch by filterfalse.