Fix __fasd_run.fish eval bug

[AbysmalBiscuit]

This PR fixes issues #16, #17, and #18.

The fix adds a function that expands variables without evaluating the passed string in the __fasd_run function.

This allows constructs such as redirections, loops, glob expansions, etc... to not be affected by the fish_postexec hook.
This lets fasd take account of relative directories.

The following is an example of save variable expansion when a for loop is passed:

$ set folders foo bar baz
$ __fasd_expand_vars 'for f in $folders; echo \$folders; cd $f; cd $abc; echo $folders; end'
for
f
in
foo
bar
baz
echo
\$folders
cd
f
cd
abc
echo
folders
end

Variables are also expanded correctly if the input is multi-line:

$ set folders foo bar baz
$ __fasd_expand_vars 'for f in $folders
  echo \$folders
  cd $f
  cd $abc
  echo $folders
end'
for
f
in
foo
bar
baz
echo
\$folders
cd
f
cd
abc
echo
folders
end