* Describe the plugin
* Show an example of what it looks like
* Describe configuration
* List license

Provide a command to print the documentation via Irssi's built-in `help`
command.

Uses the mechanism I used in Vimput to get autocompletion of "dimpid"
even though it isn't a command.

Describe the plugin and how to use it (by setting `dimpid_nicks`).

Instead of using a hard-coded nick, add a setting where any number of
nicks can be defined. The message text for these nicks will be changed
to grey.

Make this more readable by using constants in place of the actual escape
codes.

After doing some research it seemed like it was going to be way too
complicated to do this with a theme.

I was able to print the message, but would need to include the whole
rest of the line, including timestamp, nick, etc. in order to make the
line look like it was a normal message. Too much work for not enough
gain. That's not to say it isn't possible. Here's the one script I found
that appeared to be doing this:
https://github.com/irssi/scripts.irssi.org/blob/d62bb05a34ffd6d8d0f719c551d8e454880ef8e1/scripts/nm2.pl

Crazy talented stuff.

Since we really just want to turn the message text grey, use a simple
`signal_continue` and colourise the text with escape codes.

Add a signal for public messages and if the message comes from a certain
nick, print the message with special formatting. Currently the format
turns the message background to magenta.

Discovered custom themes from:
https://github.com/irssi/scripts.irssi.org/blob/master/scripts/ignorsula.pl