Changes to support htmlwidgets integration

[dmurdoch]

This PR changes two things:

1. It changes mark() to allow meta to be specified as a chunk option. This will allow an htmlwidget record_print() method to insert dependencies.

I considered some other ways to do this:
a. Use the litedown.html.meta option. The problem with this is that there doesn't seem to be a way to restore that option at the end of the run, whereas fuse() does restore reactor() values.
b. Add a new function that code could use to set options. This seems unnecessarily complicated.
c. Use Javascript methods directly. The problem with this is that some dependencies could end up inserted multiple times.

2. It changes gen_tag() so that a file dependency can be specified as an absolute path. That's how htmltools::resolveDependencies() will produce them.

It would be possible to convert those absolute paths to relative paths, but that seems kludgy.

This code works with the litedown branch of https://github.com/dmurdoch/htmlwidgets .