Rename key to custom_curl_opts

[WardsParadox]

A continuation of #112

[grahampugh]

I'll merge in the changes. I guess the key should ideally be added to the input_variables dict of all the processors, but that would mean everyone having to update their overrides, so I'll leave it be.

[WardsParadox]

They could also add it via preferences which would allow folks to add custom opts, enter it via the key value under run or use a pre-processor to add it without modifying recipes or overrides.

[grahampugh]

Yes of course, it's only to properly document the available keys.