Allow heading output to have decorated id's

[phiggins42]

Simple tiny change to allow intercepting the lexer results to
decorate the headings in the parser with potentially unique id's.

would be useful for bulk processing of a tree of .md and/or deep linking
to a section via #hash in rendered output

[Mithgol]

For future references: #173 is relatively compact, and #117 seems to offer more options.

[phiggins42]

@Mithgol yah, I saw #117 and #129 would provide the same functionality except it has been open for several months with a concern on performance. I would gladly take the small perf hit on parsing by using #129. The addition of a single ternary here gets me over the hurdle I am currently facing, the rest of the rendering intercepts would just be nice to have.

[chjj]

Closed in favor of #181.