README fix for bold CSS extension status text.
Correct positioning for cues with a negative line position.

Cues whose computed line position is negative should have line
positions relative to the bottom of the writing direction. Bottom
of the video for horizontal, or left or right of the video
depending on vertical writing direction.
Reverse step direction at the correct time.

The direction of the steps should only be reversed when
the axis along which the overlap between the container box
and the cue box is happening is the axis that the steps are
moving towards.

I've had to turn off the large-integer-value processing test
because it's hanging for a long time on the step algorithm.
Limit line cue setting to values in the -1000~1000 range.
Calculate line height by counting the lines.

This will probably have issues accounting for runs of
text that are wrapped, but it's more accurate then
the previous way of calculating the line height.
Turn ruby processing model tests back on.
Turn tests with line height of 0 back on.
Apply a padding overlay box with a margin of 1.5%.

I've also started to round the font size setting to
simplify the JSON files a bit.
Turn processing model tests that were accidentally turned off back on.

I've added a new test-runner method that tests both parsing, without
streaming, and processing whole VTT files.