Added text decoration configuration item

[goakly]

Added ability to use text decorations (italics, bold, underline, etc) via a configuration item called textDecorations.

• All decorations are disabled by default.
• A list of possible text decorations can be found [here](https://neovim.io/doc/user/api.html#nvim_set_hl()).
• When using the color palette, text decorations will apply to every color.

This is my first attempt at Lua so let me know if there is a better way to do this.

[m-demare]

Hi! There's actually an option to allow text decorations already (it's called highlight) However, it doesn't work when using the colorpalette, so I think that's a great addition. Could you change that to use the existing option?

[goakly]

Didn't realize the highlight option could do text decoration. Rewrote so that the color palette could use options set in highlight and I added a section in readme outlining the feature!

[goakly]

Actually, feel like a section in the readme about text decoration is unnecessary. Removed.