fix HRs in list items + code. fixes #91.
fix link defs after blocks. nearly fixes #77.
use renderer to customize output
fix blockcode render, more test cases passed
detail fix. 44/48 tests passed, just as the master branch
export Renderer
convenience parse api
add readme for renderer
this is not a bugfix, version bump to 0.3. remove preferGlobal, coz it is a lib more than a program
remove the convient way
global leak of flags
make example more clearly
minor async refactor.
update tests in other directories.
fix new lheading rule (2 chars) for markdown guide test.
Generate heading IDs in HTML output.

Uses the heading text itself as the ID for the H*. The only
transformation made is to replace whitespace with '-' (hyphen). This
conforms to the HTML5 spec wherein ID attributes can contain any
non-whitespace character. (The previous HTML4 restrictions on ID
attribute values have been relaxed.)

This feature is limited in scope to strictly generating heading IDs.
A future enhancement would be a simple way of letting the author specify
the ID attribute. Perhaps:
  # Some: Crazy Header  {#custom_id}

  to generate: <h1 id="custom_id">Some: Crazy Header</h1>
Fixed the parameters' order of the hightlightAuto function call in the hightlight code example
`runTests()` should return `true` if tests passed, `false` if some failed
`node test` should have non-zero exit code if some tests failed
;