Add support for setting "pipelining" to "multiplex"

[Zapotek]

I'm not sure how to go about changing the current type of :pipelining from bool to a bitmask but it should be an easy task for anyone with experience with the codebase.

Relevant code:

• https://github.com/typhoeus/ethon/blob/master/lib/ethon/multi/options.rb#L41
• https://github.com/typhoeus/ethon/blob/master/lib/ethon/curls/options.rb#L210

References:

• https://curl.haxx.se/libcurl/c/CURLMOPT_PIPELINING.html