fix: bring ITestCallbackContext in from mocha

Fixes #11

src/types.ts

@@ -1,5 +1,3 @@
-import * as mocha from 'mocha'
-
 export type PluginBuilder<I, A1 = undefined, A2 = undefined, A3 = undefined, A4 = undefined> = (arg1?: A1, arg2?: A2, arg3?: A3, arg4?: A4) => Plugin<I>
 
 export interface Context {
@@ -28,7 +26,15 @@ export interface PluginDef {
 
 export interface Plugins {[k: string]: PluginDef}
 
-export type MochaCallback<I> = (this: mocha.ITestCallbackContext, context: I, done: MochaDone) => any
+export interface ITestCallbackContext {
+  skip(): this
+  timeout(ms: number | string): this
+  retries(n: number): this
+  slow(ms: number): this
+  [index: string]: any
+}
+
+export type MochaCallback<I> = (this: ITestCallbackContext, context: I, done: MochaDone) => any
 export interface It<I> {
   (expectation: string, cb?: MochaCallback<I>): void
   (cb?: MochaCallback<I>): void