Support for space-delimited abbreviations was added to the expansion
parser in fbade198; this commit extends that support to the user-facing
tools, and documents the space-separated behaviour. Equals-delimited
abbreviations are expected to be removed before the next release.
Work on #731.
The usage is still the same, but it's a lot more robust, and also no
longer assumes $fish_user_abbreviations must be a universal variable.
This also fixes the unexpected error output when calling `abbr -a` with
no existing abbreviations.
Calling `abbr -a` with an abbreviation that already exists now silently
overwrites the abbreviation, just like `function` and `bind` do, instead
of complaining.
