`encoding` option accepted values

[ehmicky]

The encoding option accepts the same values as Node.js child_process:

• utf8
• buffer
• utf16le
• hex
• base64
• base64url
• latin1
• ascii

It also allows the following:

1. null as an alias to buffer
2. utf-8 as an alias to utf8
3. utf-16le, ucs2 and ucs-2 as aliases to utf16le
4. binary as an alias to latin1
5. Any case, e.g. UTF8 instead of utf8

@sindresorhus Do you think we should support the same options as core Node.js, or we should be stricter? If stricter, which of the above should be kept or not?

[sindresorhus]

Stricter. I would drop 1, 2, 3, 4, 5. Needs a human-friendly error when any of those are passed.

[ehmicky]

Alright, let's do this. I'll make sure the validation error message includes the value to use instead.