Move common text handling routines to vumi core

[vgeddes]

The application multiplexer (Vumi-Go) and various other services such as GFM and Wikipedia have their own hand-rolled functions for performing all sorts of text manipulations. These include functions for cleaning and stripping input, parsing menu choices, creating menus, and truncating text gracefully.

These could all be moved into Vumi core at some point, saving duplication and possible bugs.

Examples:

• Application Multiplexer vumi-go#844 (diff)
• GFM/figo/common.py

[hodgestar]

I'm not really sure these are generic enough to warrant being in core, but
if we start adding core things that use them I'm fine with them being
included.