Support using custom defined loops with storj_init_env

[braydonf]

With both the Ruby and Node.js bindings, using the UV default loop is preferred, as the loop will be managed and run automatically. We should better support this case because in storj_destroy_env it will try to close and free the loop and cause issues.

Related area of code:

• https://github.com/Storj/libstorj/blob/master/src/storj.c#L780-L782
• https://github.com/Storj/node-libstorj/blob/master/libstorj.cc#L484-L485
• https://github.com/Storj/ruby-libstorj/blob/4b0810732dc8111c49f7cef071e765e89859c7cb/lib/ruby_libstorj/env.rb#L17

Related:

• storj-archived/ruby-libstorj#2