Introduce Message::plaintextParam

Introduce Message::plaintextParam

I have run into numerous issues trying to utilize unsafe user
provided content as an argument to a Message instance.  Specific
cases are enumerated in MessageTest.php

Typically the solution to using user provided text is to use
Message::rawParam, but this pushes escaping of the parameter to
the caller.  This patch introduces Message::plaintextParams which
handles escaping of the string parameter to match the requested
output format.

The functionality is:
* plain and text: exactly like rawParams()
* escaped, parse and parseAsBlock: escape it but don't do brace expansion

Additionaly, similar to Message::rawParam, plaintext parameters are not
valid parser function arguments.

Change-Id: I320645cd23c98fea4bfc32ab22b7ef8d320957cb