aliases: git: consistent behavior for git users

The case block in git.aliases.bash does the right thing for vim users
that do not populate $EDITOR.  However vim users that populate $EDITOR
are met with the following error:

$ gd
Vim: Warning: Input is not from a terminal
Vim: Error reading input, exiting...
Vim: preserving files...
Vim: Finished.

Fix this inconsistency by adding another case block in the else block
which handles passing the appropriate arguments to vim.

Signed-off-by: Michael Turquette <mturquette@deferred.io>
plugins: extract: add new preview-extract function

It is often desirable to inspect the contents of an archive before
unpacking it.  Add a new function, preview-extract, which does this.
preview-extract is a copy-paste of extract(), but the tar commands have
parameter 'x' replaced with 't' to print the pathes of any files in the
archive.

The non-tar lines have been commented out since I rarely use those and
don't know the equivalent parameters off hand.  Hopefully a kind soul
will add those in.

Signed-off-by: Michael Turquette <mturquette@deferred.io>
plugins: trivial typo fixes

Signed-off-by: Michael Turquette <mturquette@deferred.io>
themes: new square theme

square is a mix of the existing 'minimal' and 'standard' themes without
any unicode characters that might fail to render in some terminal
emulators.

Signed-off-by: Michael Turquette <mturquette@deferred.io>