Add expression mode

In this mode, indicated by a leading "|" in the format string, the
current selection will be parsed and set to `_` in the expr env.
Supports format string and stop expr, and is also able to cast the
selection to a type beforehand. Reverse mode currently does not work as
intended.

Syntax:
exprmode      ::=  [cast] "|" [~format::][expr][@stopexpr] ["!"]
cast          ::=  "i" | "f" | "s" | "b"
Little code formatting
Rework reverse mode for exprmode

In the process: Fix bad regexps for expressions with "!" reverse at the
end since "!=" is actually a valid operator and contains "!".

Example for exprmode (also reverse mode):
1. "@p==100"
2. "i|~03::(p+3 if i!=0 else _)"
3. "i|~03::(p+3 if i!=0 else _)!"
4. The first number will be 595
Add re module to expression env
Add a menu item to open README

Also reduce complexity by using the $platform variable for keymap files.
Update README.md to all recent changes

- Stop expr
- Expression mode
- Examples and general overhaul
- Myself as contributer
Add update message