Number all code examples & add specification build to Makefile (#894)

* Merge makefile changes from dev/specification-fixes

- New top level makefile target: `specification`
  - Builds the specification document.

* Number all code examples.

This is more a change of convenience than anything. Referring to syntax
examples is much easier when they are numbered!

This commit is in the context of freechipsproject/firrtl#890 - Updating
examples and syntax specification is made easier if they are numbered.

- Change `verbatim` environments to `lstlisting`
- Add very basic keyword highlighting.
- Rebuild specification PDF.

 On branch dev/number-code-examples
 Changes to be committed:
	modified:   spec/spec.pdf
	modified:   spec/spec.tex

(cherry picked from commit 29e5c088)