main: redraw every time a file loads.

Eliminates lag time between a file load and a redraw.
Add more info to readme.
Refactor display into OSDAggregator class.

The OSDAggregator can handle elements from multiple modules. Because
mp.set_osd_ass can only handle displaying a single "script" at a time,
this resolves some of the inherent limitations caused by that
restriction.

This commit also (attempts) to rice the hell out of the ProgressBar
display process by avoiding formatting very large strings. The longer a
format string is, the longer it takes lua to copy all the memory
around. In tests with luajit, short format strings combined with
table.concat proved to be much faster.