Barnhunt

- Remove tags from layer.label when expanding templated text in SVG file.

- Add `--version` command line option

Pager for `coords`

- A fancy pager (poor man's `less`) has been added for viewing the
output of the `barnhunt coords` sub-command. If any of `sys.stdin`
or `sys.stdout` is not a tty, then the pager will be disabled.

- Since there is now a fancy pager, the default for `--number-of-rows`
has been increased to 1000.

Things still to be fixed

Things still to be fixed: I'm pretty sure things are direly broken if
a drawing contains no overlays, and somewhat broken if a drawing
contains more than two layers of overlays. The problems have to do
with how the output PDF filenames are determined...

New layer flag scheme

New scheme for marking overlay and hidden layers. One can now set
bit-flags on layers by including the flags in square brackets at the
beginning of the layer label. I.e. a label like `"[o] Master Trial
1"` marks the layer as an overlay layer, while `"[h] Prototypes"`
marks a hidden layer.

If no layers have any flags, `barnhunt pdfs` will fall back to the
old name-based heuristics for determining hidden and overlay layers.

* When exporting PDFs, run `inkscape` with `--export-area-page`.

Packaging

* Fix MANIFEST.in. Tests were not being included in sdist.

* Add `url` to package metadata.

* Ignore *ring* layers when identifying *course* layers. (Now a layer
labeled “C8 Ring” will not be treated as a course layer.)

* `pdfs`: default `--output-directory` to `.` (avoiding exception when no
explicit output directory is specified.)

* Change `barnhunt coords` so that it omits duplicate coordinates in its output.
Also increase the default for `--number-of-rows` to 50 and
add the `--group-size` parameter to separate output into groups.