Added Base85 operations (new)

[PenguinGeorge]

To resolve #286.

This adds two new operations:

• To Base85
• From Base85

Both operations include three common alphabets for Base85:

• Standard - also known as Ascii85, and is the standard alphabet for base85.
• Z85 (ZeroMQ) - a "string safe" variant which doesn't use single or double quotes, backslashes, or other problematic characters.
• IPv6 - alphabet used for encoding IPv6 addresses, as defined by RFC 1924.

The "To Base85" operation also has an "Include delimiter" option, which will add <~ and ~> at the start and end of the output, which is a standard used in Adobe PostScript.

[CLAassistant]

All committers have signed the CLA.

[n1474335]

Thanks very much @PenguinGeorge, this is really well written. I updated the alphabets to use ranges in the same way as the Base64 ops. This is much more succinct. I also changed some of your string concatenation to use template strings. Other than that it's as you wrote it.