Implement Consistent Color Generation (XEP-0392)

This should make colors used by Delta Chat for emails similar to colors
used by XMPP clients implementing the same specification[1], such as
Conversations and Snikket.

Previously Delta Chat used only hardcoded 16 colors, so this change
should also increase the number of colors and make it easier to
distinguish different contacts.

[1] https://xmpp.org/extensions/xep-0392.html

src/lib.rs

@@ -77,6 +77,7 @@ pub mod stock_str;
 mod token;
 #[macro_use]
 mod dehtml;
+mod color;
 pub mod html;
 pub mod plaintext;