Close #115

This implementation seems to match the behavior of the previous iron
server.

Static files are rendered as-is, and when a directory is requested, we
attempt to render a `index.html` found inside, or 404 if it's not
present.

The actix docs mention using channels to send a message to the server to
shut it down gracefully while running in another thread (as we're doing
here), but it seems like there would have to be some reorganization in
order to manage this effectively, perhaps holding the channel sender inside
`main.rs` so we can push a message through to the server when the call
to `cmd::serve()` finally returns.

For the time being, I left things without any careful attempts to
cleanup the server thread. This more or less matches the old iron
implementation as far as I can see.

The static file handling in actix is _just_ a little off from what we'd
want.
I left some comments in the source regarding why we can't just use their hook
for directory index redirection.

Closes #277

Closes #272

Closes #254

Closes #238

* Add ignored_content to the Config structure.
* Use the GlobSet crate to parse the glob patterns into a matcher, which
  is created once at program initialization. If there are no patterns in
  ignored_content, an empty globber is created, which excludes no files.
  This is consistent with the existing behaviour of Gutenberg, before
  this feature was added.
* Bail if there are any errors in the glob patterns.
* Add a call to the globber in page.rs to actually do the filtering.
* Update documentation.

A note on the Config structure
------------------------------
* I had to remove the PartialEq derive from the Config structure as it
  does not work for the GlobSet type. No harm is done, Config does not
  need to be PartialEq anyway, since there is no need to sort Configs.
* The implementation follows the pattern of the existing config settings
  in that it uses an Option<...>. This would appear unnecessary, in that
  an empty vec could be used as the default, but it appears to be needed
  by the TOML parsing. A better approach would be to use a separate
  SerializableConfig and map to/from a Config struct. This would also
  allow the elimination of most, if not all, of the other Options in
  the Config structure, but that ought to be another PR.