Add options for which case to use in Base functions

[ericmj]

No description provided.

[pma]

+1 for lowercase. I'm using Base.encode16(string) |> String.downcase almost all the time, just because I think lowercase hex "looks" better.

Regarding the way we pass the lowercase option, please consider that other options may be required in the future. Many encoding libs also include options to omit the padding in Base64/Base32 or to set a padding char different than ?=. One option is to use a keyword list as mentioned in #2143.

Allowing mixed case when decoding should be avoided in my opinion. We can always do a previous String.downcase/upcase if we really want the mixed case behaviour. But as stated in chapter 12 of the rfc (http://tools.ietf.org/html/rfc4648#page-14) there are security implications.

[josevalim]

👍 for making the case an option instead of an atom: case: :lower. @ericmj can you please amend?

[ericmj]

Pushed with discussed changes.

[josevalim]

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