Support wide characters and true color
1. I'm happy to divide this into two issues (one for wide chars, one for true color) if that's preferable.
2. I'm offering to submit PRs, but want to know if you'd rather depend on the library I've made for these features, or fold them into illwill itself.

## Wide Characters

On macOS Terminal (and I assume other terminals), the current version of illwill assumes that all characters occupy one space.  Emoji and some other characters actually occupy 2 spaces.  For aligning tables and boxes, it's important to know how wide strings are.  For example, see that the smiley below covers 23 and the last two characters each cover 2 spaces:

<img width="357" alt="Screen Shot 2020-10-27 at 4 07 12 PM" src="https://user-images.githubusercontent.com/161727/97356336-7f87af80-186e-11eb-8e5b-edca0b7d86a7.png">

## True Colors

Some terminals support 16 colors, some 256 and some 24-bit "true color."  It would be nice if illwill did, too.  Then you can do things like this:

Windows CMD prompt:
<img width="207" alt="Screen Shot 2020-10-27 at 4 02 56 PM" src="https://user-images.githubusercontent.com/161727/97356786-266c4b80-186f-11eb-82a2-d81a24294cbe.png">

Windows Terminus (lacks 256 color support):
<img width="235" alt="Screen Shot 2020-10-27 at 4 03 16 PM" src="https://user-images.githubusercontent.com/161727/97356823-37b55800-186f-11eb-9dd3-a93ef9cdd3e2.png">

macOS Terminal:
<img width="576" alt="Screen Shot 2020-10-27 at 4 03 42 PM" src="https://user-images.githubusercontent.com/161727/97356835-3dab3900-186f-11eb-9412-0bdf29f8cb41.png">

---

I've looked at how other libraries work around this and have come up with: https://github.com/iffy/termtools  It contains [methods for detecting wide char string lengths](https://github.com/iffy/termtools/blob/master/src/termtools/runewidth.nim#L92) and [color escape sequence generation.](https://github.com/iffy/termtools/blob/master/src/termtools/color.nim#L148)

I think my preference is to depend on this new library as others might want the widechar and color support it provides without making a curses-like UI that illwill provides.  But I'm interested in your thoughts before I proceed.