1. When run with no arguments, make abbr do the equivalent
of `abbr --show`
2. Enable "implicit add", e.g. `abbr gco git checkout`
3. Teach `abbr --show` to not use quotes for simple cases
4. Teach abbr to output -- when the abbreviation has
leading dashes
Add some basic tests to abbr too.
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.
