Add support for custom key macros

[ulyssa]

This adds a "macros" field to config.json which allows specifying custom mappings from one set of keys to another. This allows writing something like the following to map jj to <Esc> in Insert mode:

{
    ...,
    "macros": {
        "i": { "jj": "<Esc>" }
    }
}

The following names can be used to refer to the different modes:

• normal/n for Normal mode
• insert/i for Insert mode
• visual/v for Visual mode
• command/c for Command mode
• select for Select mode
• operator-pending for Operator Pending mode

Multiple modes can be specified together using | to put a mapping in all of them. For example, "insert|command" or "i|c" could be used in the above example to map jj in both Insert and Command mode.

I've called this "macros" since they'll behave like macros: a {count} argument will repeat the mapping (<Esc> in the above example), and not provide an argument to it. I have plans for a separate way to specify keybindings in the future using expressions that generate an Action that should help anyone who needs to specify something that takes an optional count or register argument.