chore: implement initial bot with substitute command

This commit introduces a Bot struct which is the main type of this
project. The Bot holds a backlog of messages to be able to update a
typo in a message and so on.

So far there is no IRC/TLS implementation, it is only the base logic.

README.md

@@ -0,0 +1,21 @@
+# zigeru
+
+zigeru is a IRC bot which implements the following commands:
+
+- `s/OLD/NEW/` -- posts the previous message by the user, where every
+  occurrence of OLD is replaced by NEW.
+
+## Getting started
+
+To enter into a development shell with all the tools needed for
+interacting with this project, run the following command:
+
+```
+$ nix develop
+```
+
+To run the tests: `zig build test`.
+
+To run the binary: `zig build run`.
+
+To build the binary: `zig build`.