Enable docs for interface CallSignatures (Fixes #123)

[AvremelM]

Allows generating docs for call signatures of interfaces and types, e.g.

    interface Foo {
        /**
         * @description
         * @param {string} bar
         * @returns {*}
         * @memberof Foo
         */
        (bar: string): any;
    }

This fixes issue #123

[joelday]

My apologies for the delay. Can you please catch up with master and test this for any regressions? Thank you!