Allow sections tagged with comments

[rakyll]

I forked the tool over the weekend to modify the behavior to embed tagged sections from files.

// START tag
... code snippet here
// END tag

can then embedded by the markdown files. An example of this behavior is at https://github.com/rakyll/embedmd/tree/master/sample. If embedmd can support this use case, it would enable the cases where regex is not enough to target.

[rakyll]

/cc @campoy