Support TERM=tmux-256color for italics

[HaleTom]

While screen-256color is supported, setting TERM to this value disables italic fonts.

Italics only work correctly with tmux-256color.

The tmux FAQ contains:

set -g default-terminal "tmux-256color"

Can this TERM be supported to allow for recommended tmux configurations generally, and also to support italics?

[HaleTom]

% infocmp tmux-256color screen-256color

This tells me that tmux also supports "has status line" while screen doesn't.

[gdamore]

tcell makes no use of the status line.

[gdamore]

Because of #252 I'll probably wind up holding off this and other new database entries -- I'm going to have to redesign the database entirely to address go modules any way.

[HaleTom]

All good.

I'm not too bothered with the status line, for me the italics are higher priority as they affect the data rather than the metadata, so to speak.

[walles]

@gdamore, now that #252 is done, how much work would it be to add support for tmux-256color?

[gdamore]

We should definitely add the tmux definitions.

[gdamore]

Closing in favor of #372