Cleanup outline before export

[louisremi]

There's a large number of best practices related to glyph outlines that can be used before exporting the font:

• moving points to the lowest/highest/leftest/rightest point of a curve (see this comment of Loïc)
• removing superposed points
• making lines out of curves with 0-length controls
• joining consecutive lines that are aligned

This would be "nice to have", but isn't required for 1.0. We should introduce some hooks that would allow plugin authors to process glyph outlines before they are exported, and this feature could be developed as a plugin.