Add .xpm support for <icon> via libXpm.

* Adds a new flag with_xpm to enable compilation with xpm support
* Adds a module XPMFile only exporting readXPMFile which almost mirrors
  Graphics.X11.Xlib.Misc.readBitmapFile
* During loadBitmap a file is first tried with readBitmapFile and if it
  fails with readXPMFile
Add the header to XPMFile
Mention XPM support in readme.md when <icon> is introduced
Merge branch 'xpm' of https://github.com/projedi/xmobar
Missing XPMFile module added to xmobar.cabal
mtl 2.2.1
News and readme update