Add new search addon, move AddonManager to public

Because addons build on top of the API it needs to live in public, the main
reason for this is because the implementation of buffer differs on
public/Terminal and src/Terminal.

src/Types.ts

@@ -220,6 +220,7 @@ export interface ITerminal extends IPublicTerminal, IElementAccessor, IBufferAcc
   showCursor(): void;
 }
 
+// Portions of the public API that are required by the internal Terminal
 export interface IPublicTerminal extends IDisposable, IEventEmitter {
   textarea: HTMLTextAreaElement;
   rows: number;
@@ -268,7 +269,6 @@ export interface IPublicTerminal extends IDisposable, IEventEmitter {
   setOption(key: string, value: any): void;
   refresh(start: number, end: number): void;
   reset(): void;
-  loadAddon(addon: ITerminalAddon): void;
 }
 
 export interface ITerminalAddon extends IDisposable {