So far only the detection of local embeds has been implemented. As this requires access to some additional properties, many tests are broken for the time being

See #267

The internal embed handler now requires access to some additional request properties that were not
mocked in all test cases

for #267

#267

…tions

The VDOMTransformer's `handler` functions did not allow for recursive processing of child nodes so
far, i.e. when a handler function wanted to process only a single MDAST node, but hand the
processing of all MDAST child nodes back to the original transformer, it had no way of doing so,
making each `handler` function a terminal node. With this change, the signature of `handler`
functions changes to `handler(callback, node, parent, handlechild)` where `handlechild(callback,
childnode, mdastparent, hastparent)` is a callback itself that takes the current HTAST-constructing
`callback`, the MDAST `childnode` that should get transformed, the `mdastparent`, i.e. the current
node from the vantage point of the `handler` function, and with `hastparent` a HAST parent node that
the output of the child processing will be attached to.

Until now, a failed ESI include would fail silently, without a possible fallback. This change falls
back to the markdown rendered as HTML in case the ESI does not work, giving at least an
approximation of the content created.

fixes #267