RST Reader: Warn about skipped directives

move `addWarning` to Parsing.hs, so it can be used by Markdown & RST readers.
expose warnings from RST reader; refactor

This commit moves some code which was only used for the Markdown Reader
into a generic form which can be used for any Reader.  Otherwise, it
takes naming and interface cues from the preexisting Markdown code.
RST: literal role should produce Code,

code role should have "code" class.

http://docutils.sourceforge.net/docs/ref/rst/roles.html says that
`text`:literal` is the same as ``text``.  docutils outputs a <literal>
element in both cases, whereas for the code role, it outputs a <literal>
element with the "code" class.
RST reader: improve support for custom roles

- Add "sourceCode" to classes for :code: role, and anything inheriting
  from it.
- Add the name of the custom role to classes if the Inline constructor
  supports Attr.
- If the custom role directive does not specify a parent role, inherit
  from the :span: role.

This differs somewhat from the rst2xml.py behavior.  If a custom role
inherits from another custom role, Pandoc will attach both roles' names
as classes.  rst2xml.py will only use the class of the directly invoked
role (though in the case of inheriting from a :code: role with a
:language: defined, it will also provide the inherited language as a
class).
WIP: tests for RST roles
RST Reader: compute Attrs when role is defined

Move recursive role lookup from renderRole to addNewRole.  The Attr value
will be the same for every occurance of this role, so there's no reason
to compute it every time.  This allows simplifying the
stateRstCustomRoles map considerably.

We could go even further, and remove the fmt and attr arguments to
renderRole, which are null except for custom roles.
RST: warn about ignored fields in role directives
Merge pull request #1805 from bergey/rst

RST Reader - Improved Role Support