v5.1.0

Prior to v5.0.0, I had transpiled Penpal code to es5 before packaging for npm. Starting with v5.0.0, I decided to only bundle for es6 with the hope that bundler support for es6 would be strong enough that packaging es5 would be unnecessary and consumers could configure their bundler to transpile penpal back to es5 if necessary. Due to popular demand and because bundlers can be a pain to configure to transpile penpal to es5, I added back transpilation to es5.

The npm package now includes both es6 and es5 code. If the consumer's bundler is capable of bundling es6 code, it should use the es6 code automatically. If it's not capable of bundling es6 code, then Penpal's es5 code will be used automatically instead.

This should not affect the runtime behavior of Penpal in any way.