Revert switch to TS 2.8's Exclude (#96)

`Exclude` is a new language feature from typescript 2.8 that replaces the
`Diff` functionality previously backing the `Omit` type.  This only
works in clients using 2.8, so let's keep using the old version for a
little while until we can be confident most people have upgraded. Or
until we can figure out how to have some sort of conditional typecheck
in typescript.

Author: wadetandy

package.json

@@ -1,6 +1,6 @@
 {
   "name": "jsorm",
-  "version": "1.3.0",
+  "version": "1.3.1",
   "description": "Javascript ORM",
   "main": "lib/index.js",
   "module": "lib-esm/index.js",

src/util/omit.ts

@@ -1 +1,4 @@
-export type Omit<T, K> = Pick<T, Exclude<keyof T, K>>
+// export type Omit<T, K> = Pick<T, Exclude<keyof T, K>>
+export type Diff<T extends string, U extends string> = ({ [P in T]: P } &
+  { [P in U]: never } & { [x: string]: never })[T]
+export type Omit<T, K extends keyof T> = { [P in Diff<keyof T, K>]: T[P] }