Fix `injectHints` might return undefined markers

The `inner` function of `injectHints` now returns an empty array instead
of undefined if no markers could be produced, since all results of the
`inner` function are recursively concatenated. VimFx seems to have worked
well even though some entries in the array of markers returned by
`injectHints` could be undefined. However, on the Huffman branch, the
undefined entries sometimes caused a TypeError to be thrown, which
eventually resulted in that none of the hint markers were given any hints.
Fix typo

Line 64 in marker.coffee contains a reference to `@markedElement`, though
that is the only occurance in the entire files. It clearly looks like it
is a typo of `@markerElement`, by inspecting the nearby code. The odd
thing, though, is that that line seems to have been added by commit
b5f488df, and already there the typo exists. Has the code ever worked?

I'm not sure if this fix currently fixes any issues, but it is needed in
the Huffman branch in order not to throw errors.
Merge pull request #147 from lydell/bugfixes

Bugfixes