When opening a new VT apply default character attributes

[martin-hughes]

Previously, if I do something like this:

TMT* vt = tmt_open(80, 25, normal_callback, nullptr, nullptr);
tmt_write(vt, "hello", 5);
ASSERT_EQ(tmt_screen(vt)->lines[0]->chars[0].a.fg, TMT_COLOR_DEFAULT);

Then the assertion will fail as fg == 0. (The meaning of ASSERT_EQ is the same as in googletest). Hopefully the obvious fix of setting the defaults in tmt_open is OK.

I've also fixed a few typos in the README.