dartdoc always parses full markdown even when it doesn't need to

[jcollins-g]

Partially because markdown doesn't currently have an option to do otherwise, dartdoc has an issue where we always generate the full HTML for each ModelElement's comment via markdown, even when we don't need to (for example, only the oneLineDoc is used). This mattered less before 0.9.11 and canonicalization as we weren't as picky about what docs we write, but now we're generating and discarding a lot of HTML, repeatedly. Profiling suggests this is why complex packages are slow, because there is a lot of inheritance and a high percentage of non-canonical ModelElements.

Given that we have to run some of the markdown for non-canonical ModelElements so we can have one-line doc summaries, I propose:

1. adding a markdown feature to only return HTML generated from the first node and using that for oneLineDoc
2. change macros so we don't need to parse all docs in case one exists

[jcollins-g]

A prototype of #1 suggests that that cuts generation time by another 20% for flutter and drops about half of the comment reference lookups during generation. A nice side effect is a squelching of some warnings for docs we never actually write to disk because they weren't part of a oneLineDoc.