Implement environment minipage with all four parameters

[cspiel]

tl;dr: minipage env now works with Hevea; layout still sucks.

Modern LaTeX allows for up to three optional parameters to be passed
to the environment minipage besides the mandatory argument width:

• position,
• height, and
• content-position.

This P/R implements a faithful translation for Hevea, which works for HTML
and text output modes.

The new minipage plays well with HTML flow, in particular it displays as
inline-block, but the fundamental problem of minipages not behaving as
"words" prevail. See the attached sample file minipage-example.zip that
exercises almost all old and features.

To integrate manipages into the HTML flow and thus closely mimick the
LaTeX behavior more code is necessary. I'm working on a prototype.