git-icdiff tweaks

1. Fix quoting of arguments - they were being captured inside the quoted string,
and not being passed to the `git difftool` command.
1. Use the user's preferred pager setting (I like `less -FXR -x4`)
Use eval to properly handle core.pager values that include backticks.
Use $() rather than backticks to invoke `git config`.
Merge pull request #24 from floatplane/fix-git-icdiff-quoting

git-icdiff tweaks