Add "caveats" to pex info

[jacobsvante]

Similar to homebrew it would be nice to have some "caveats" info regarding the installation of an extension.

For example I was unable to install postgis as my brew package gettext wasn't linked (it's keg-only). This would be a great addition to a caveats section.

I'm imagining an output like this when running pex info postgis:

name: postgis
homepage: http://postgis.net/
url: http://download.osgeo.org/postgis/source/postgis-2.2.2.tar.gz
status: installed
caveats:
- If you're getting an error similar to fatal error: 'libintl.h' file not found you may have to install gettext with homebrew and link it with brew link gettext.

[petere]

I had thought about that, but pex isn't specific to OS X, so the caveats would not need to be tailored to the platform somehow. So it will need a bit more work.