only allow html files in the sitemap.xml

The sitemap is for search engines, so we should probably only have HTML
files inside.

Fixes #176
ignore hidden files in the static app

This is mostly to ignore Vim swap files, but in general, hidden files
should not be moved by default.

Fixes #174
rename Store to Store::File

This opens us up to more stores in the future, and also lets us do some
file-specific things we want to do.
make all file stores ignore other stores' files

Each file store that is created should ignore the files/documents of all
other file stores that are created. This problem happens most often if
you're nesting file stores, like:

    /               # Site root source for Plain app
    /static         # Source for Static app
    /_build         # Destination for build

If the /static store has any documents, they'll be read by the Plain
app, and also placed in the /_build destination. Next time the Plain
app reads its documents, it now also finds documents inside /_build.

Fixes #172