Improved handling of escape sequences

[Grimy]

This is a followup to #66.

• The Paint gem was emitting a \e[0m sequence after each painted character. This resets all graphical attributes, including background color, bold, italics… As a result, whenever a run of characters was supposed to be highlighted, only the first character in the run was actually highlighted. Example from htop:

To fix this, we instead use \e[39m to reset only the foreground color (or, in invert mode, \e[49m to reset only the background color). This gives much prettier results:

• Some things still weren’t properly reset. The aptly named stty sane does most of what we want, so we now use that.

• We now detect incomplete UTF-8 characters and incomplete escape sequences, and perform another read when they occur, thus fixing the regression that was mentionned in Make nethack | lolcat work #66.

[m-o-e]

"In the beginner's mind there are many possibilities, but in the expert's there are few."
-- Shunryu Suzuki

Merged and released.