Add a read-only option to the edit widget

Thanks again to @Peter2121 for this suggestion and implementation!

If the edit widget is set to read-only, user input that would otherwise
change the content of the widget is short-circuited. Note that this
still allows scrolling around the widget.

Add a new paragraph widget constructor

This allows the user to pass the individual words individually, if the
split algorithm that New() uses isn't appropriate.

Better color interpolation when base16-shell is in use

If your gowid application expresses its colors in RGB
format (e.g. #aa55bc), and your application is running in a 256-color
terminal (without COLORTERM=truecolor set), then gowid will try to find
the closest matching color in the 256-color space. If you are using
base16-shell (https://github.com/chriskempson/base16-shell), then your
base16-shell theme likely has remapped colors 0-21 out of the terminal's
256-color space. Gowid doesn't know this, and assumes colors 0-21 are
"normal". This can result in it interpolating a poor match for the
user's desired RGB color.

You can set IgnoreBase16 to true to have gowid ignore colors 0-21 when
selecting the best match for your RGB color. If you set
GOWID_IGNORE_BASE16=1 in the environment before running, then your
application will apply this rule automatically. For example, this would
mean black - RGB #000000 - would map to color 232 in the 256-color
space (https://jonasjacek.github.io/colors/)

Note that this is a global variable, not set at the application level.

Be more forgiving if style types are nil.

If the user creates a styled widget like this

styled.New(w, nil, nil)

then don't crash when rendering - just don't style it.

Gowid 1.0.0