feat: add option to lookup from env variable

[darkgnotic]

As requested and specified in #30

An Option's value, in order of increasing precedence, is taken from:

• the Option's defaultValue
• the value from a config file
• the value from the Option's env variable 🆕
• the value from the command line

Values from process.env are parsed as if they were specified on the command line.

[darkgnotic]

@Roaders I took a stab at #30. Thanks for maintaining this cool library!

[Roaders]

Thanks for the PR. I will try to get round to taking a proper look at it soon.