Add formula for pidgin (also installs finch)

This formula installs the Pidgin instant messaging client with the
majority of its dependencies. Some are not currently packaged for
Homebrew and make arrive later. These are listed below.

Currently this formula requires xgettext and other binaries to be in the
PATH during installation. This may require the following steps:

$ brew link gettext
$ brew install pidgin
$ brew unlink gettext

Unsupported dependencies:

- Farsight (for audio/video chat)
- D-Bus
- Avahi
- Meanwhile
- GtkSpell
- NSS (GnuTLS is used for SSL suport)
- Gstreamer
- libidn

Closes #13901.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
pigdin 2.10.7
pigdin needs pkg-config