Add begin.convert() decorator

This decorator converts types of arguments so that functions decorated
by begin.start() are not limited to accepting string data types.
Make metavar in usage help be env variable name

The metavar name used in the generated usage help will now reflect the
environment variable used to set the option's value.
Add Flask and Bottle based examples

Adapt the example applications for both Flask and Bottle from their
respective quickstart guides to use begins.