rework var formats to match gettext var formats

[willkg]

The format for variables in a given string can be encoded in the #, (pound comma) comments in th e.po file on a msgid by msgid basis.

http://www.gnu.org/software/gettext/manual/html_node/PO-Files.html#PO-Files

Two things:

1. we should use the same names that gettext does (e.g. c-format, python-format, etc) when specifying variable formats
2. dennis should be clever enough to have a default to use when no #, is specified for that msgid and to use the #, flags specified by the msgid when available

I think the immediate fix is this:

1. change "Var" to "Format" (and cleanup related things)
2. split "pysprintf" into "c-format" (e.g. %s) and "python-format" (e.g. %(foo)s)
3. change "pyformat" into "python-brace-format" (e.g. {foo})

This probably affects lint rules, too.

Also, we should cover this in the dennis documentation with links to the gettext docs.

[willkg]

I'm going to do c-format in another bug since there are some transposition things it does that I need to look into. That's issue #58.

Handling #, is covered in bug #59.