Support for optional prefix

[urish]

Allow specifying a prefix that will be added in front of every type automatically discovered. For example, if the given prefix is /*auto*/, and the input source file is as follows:

function f(n) { return n + 1; }; 
f(5);

the result would be:

function f(n: /*auto*/number) { return n + 1; }; 
f(5);

This can also be useful for toggling errors about the discovered types all at once. First, define a new type with a distinct name that maps to void. Then use this type name, followed by the pipe character (|) as a prefix. for instance, if you chose the name the type AutoDiscovered, then you declaration would look like:

type AutoDiscovered = void;

and you will use the value AutoDiscovered| for your prefix. Then, you will be able to silence all TypeScript errors introduced by the discovered types by simply changing the declaration above to read:

type AutoDiscovered = any;

Then you can toggle these errors on/off by simply switching the declaration of the type between any and void. Thanks @benland for this idea.