Support union type as index signature parameter type.

[saml]

Given:

interface Foo {
  [key: string | number]: boolean;
}

I get

bleh.ts(2,4): error TS1023: An index signature parameter type must be 'string' or 'number'.

A workaround is:

interface Foo {
  [key: number]: boolean;
  [key: string]: boolean;
}

But, I need to match up the type of value, boolean. Since it's a repetition, it could be useful to programmers to specify index signature parameter type as string, number, string | number, or number | string.

[danquirk]

If your string and number indexers are the same then you don't need to do this. A string indexer subsumes the numeric indexer. If there's something more involved for indexer types you needed maybe #2049 is relevant.