Export sanitizers for easier use

[mattbishop]

The current out-of-the-box sanitizers are great, but a bit odd to use because you have to declare a new function to access them. It would be easier if they were exported, so this:

const useLocal = env("APP_ONLINE", x => x.asBoolean(), true)

becomes this:

const useLocal = env("APP_ONLINE", env.asBoolean, true)

Or, even easier:

const useLocal = env.asBoolean("APP_ONLINE", true)

[tal952]

Hey,
I really like:
const useLocal = env.asBoolean("APP_ONLINE", true)

The only problem is that We could not chain operations, but chaining is not that important.
The standard usage will benefit greatly with a syntactic sugar like this.
Would you like to contribute a PR?