fix: hardcoded '/' in normalized options.fs, fixes #412, #408

Changes including:
- will not call fs.resolve when normalizing `fs`
- removed hardcoded `/`
- mandatory `fs.dirname` for relative reference
- mandatory `fs.sep`, defaults to '/'

src/util/assert.ts

@@ -1,8 +1,10 @@
 import { AssertionError } from './error'
 
-export function assert <T> (predicate: T | null | undefined, message?: () => string) {
+export function assert <T> (predicate: T | null | undefined, message?: string | (() => string)) {
   if (!predicate) {
-    const msg = message ? message() : `expect ${predicate} to be true`
+    const msg = typeof message === 'function'
+      ? message()
+      : (message || `expect ${predicate} to be true`)
     throw new AssertionError(msg)
   }
 }

src/util/underscore.ts

@@ -11,6 +11,10 @@ export function isFunction (value: any): value is Function {
   return typeof value === 'function'
 }
 
+export function escapeRegex (str: string) {
+  return str.replace(/[-/\\^$*+?.()|[\]{}]/g, '\\$&')
+}
+
 export function promisify<T1, T2> (fn: (arg1: T1, cb: (err: Error | null, result: T2) => void) => void): (arg1: T1) => Promise<T2>;
 export function promisify<T1, T2, T3> (fn: (arg1: T1, arg2: T2, cb: (err: Error | null, result: T3) => void) => void): (arg1: T1, arg2: T2) => Promise<T3>;
 export function promisify (fn: any) {