Support custom keybindings

[Tyriar]

Raised from a discussion in #107, I would like to eventually let vscode users to be able to configure keybindings for:

• Copy and paste (Support copy and paste via keyboard on integrated terminal microsoft/vscode#6451, this one's particularly urgent)
• Clear terminal (⌘K on OS X [terminal] (⌘K) was pressed. Waiting for second key of chord... microsoft/vscode#7240, general command request Terminal: provide an action to clear the terminal microsoft/vscode#6603)

What I'm thinking is allowing consumers of xterm.js to be able to plug key -> escape sequence mappings into evaluateKeyEscapeSequence. This could return the same result interface if there is a match otherwise undefined.

[parisk]

I was thinking that an interesting solution would be offloading the copy/paste functionality to an addon, so as to be able to apply any functionality programmatically on top of the core terminal.

[Tyriar]

@parisk exposing some API like Terminal.prototype.registerKeyMappings(event) would be good, then that can be used by both addons for common customizations like copy and paste and also lets consumers do whatever they want with it.