Add service to allow other plugins to open terminal

[edjubuh]

Closing #77 in favor of this.

This PR adds the ability to allow other plugins to consume a terminal-tab service and create Terminals using this plugin.

Some use cases:

• ide-powershell (uses a fork of this plugin to consume a service to create a PowerShell console)
• purduesigbots/pros-atom3 (would use this service to create a serial console to a microcontroller)

[edjubuh]

From: #77 (comment)

Providing a service is certainly the preferred option here. However, I want to proceed very cautiously due to the obvious security concerns.

This is a terrible excuse, but exposing xterm to other plugins on Atom doesn't feel like much more of an security concern because Atom doesn't treat it as a threat. If a user has been infected with malicious code running inside Atom, there are other facilities to run arbitrary programs/execute arbitrary scripts (BufferedProcess, ChildProcess).

[edjubuh]

Any update here?