Fix runaway memory leak when expanding invalid tags, and remove error

Don't recursively parse on the case of an error. Also, in the case of an
error, instead of replacing by an error message, replace by the tag
itself to prevent mangling strings that legitimately include < and >
characters, for example song titles when using the MPD plugin.

I'm not sure how to handle this better. Honestly, I would like to remove
the recursive parsing altogether but this works for now. Consider filing
an issue for a proper resolution of this. I don't want rogue tags
expanding themselves in my song titles.
Doc fixes
Add support for multiple actions per item, activated depending on mouse button clicked
Change actions syntax
Update readme
Credits for Marcin Mikołajczyk
have disabled plugins still accept input

This change lets xmobar compiled without -fwith_inotify accept a config
that mentions the Mail plugin with a warning (and display an
Updating...).
NEWS update
src/Plugins/Monitors/Volume.hs: port to alsa-mixer-0.2

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Ignored Sandbox
Handle inexistent devices

 If a device is unplugged without prior unmounting, it still
 appears in the /etc/mtab file, which is used for this monitor.
 The problem is that this monitor canonicalized every path
 regardless of whether it actually existed or not. I basically
 added the 'only canonicalize if file exists'.
Merge remote-tracking branch 'rethab/master'
News update
Merge remote-tracking branch 'upstream/master'