styles-demo is inaccurate/out of date

Calling `:styles-demo` appears to simply dump the file share/styles-demo.txt. It seems to be missing some styles and doesn't accurately demonstrate some styles.

Here's a screenshot of styles-demo:

Here's a screenshot generated with the small script at the bottom of styles demo. (In this case, I tested with `git-pull` and changed `head` to `tail` since the first five lines of the output were all comments)

Two things of note:
1. the format of the comments on the rainbow_dash is different
2. the sas style is missing from the styles-demo output

Some thoughts on resolution:
* update shares/styles-demo.sh
* change `:styles-demo` to return all styles dynamically, using a representative command output
* change `:styles-demo` to be a query parameter so one could do `curl cheat.sh/git-pull?styles-demo` and the get dynamic output for all existing styles, like in the script

_Edit: FWIW, I'm on MacOS using [iTerm2](https://iterm2.com/) with a light background_

Start with shares/styles-demo.sh and the static share/styles-demo.txt output, then inspect how :styles-demo is assembled. Compare the static and dynamic demonstrations, including the rainbow_dash comment format and the missing sas style. Done means the styles demo accurately represents all existing styles or clearly implements the selected resolution.

Written by the indexing model from the issue text.