fixes#526 tcell emits redundant attributes
This work is inspired by, and partly derived from, work submitted by
Simon Ser (@emersion). However, we've modified the bottom half of
the terminfo parser to better support strings properly, and are using
proper terminfo syntax.
Instead of an attribute called Hyperlink, we have called it Url
for the sake of brevity.
While here we noticed and fixed bug #526, which could badly impact slow
terminals, or slow links. This likely makes things better for folks
coming over long distance SSH connections for example.
We've also provided OSC 8 handling for all terminals which appear to
support the mouse sequences; hopefully ones that don't handle this
sensibly will just ignore it. (Limited testing seems to show this.)
Garrett D'Amore