Add an API for specifying terminal location

[Tyriar]

Here are two options:

export interface TerminalOptions {
  splitFrom?: Terminal
}

or

export interface Terminal {
  split(options: TerminalOptions): Terminal;
}

/cc @fabiospampinato

[fabiospampinato]

They both look pretty good to me.

Maybe I'm leaning a bit toward the latter, especially if split direction and shell/args/env inheriting were to be implemented in the future, I think this:

export interface TerminalSplitOptions {
  direction: string,
  inherit: boolean
};

export interface Terminal {
  split(options: TerminalOptions, splitOptions?: TerminalSplitOptions): Terminal;
}

Looks a little nicer than this:

export interface TerminalOptions {
  splitFrom?: Terminal
  splitDirection?: string,
  splitInherit?: boolean
}