Implement the index/settings forwardToReplicas query parameter (pull request available)

[quaaantumdev]

This is available as a non-breaking pull request and simply be merged: #135

Is your feature request related to a problem? Please describe.
When changing the settings of an index, it seems rather important to also forward these settings to all replicas. The option however is not yet implemented by the library. This most will most likely result in an inconsistent configuration across replicas.

Describe the solution you'd like
The option to set forwardToReplicas

Describe alternatives you've considered
Quitting software development, going back to the Stone Age. However, it seems like most people won't follow me there. The resulting low competition will increase the likelihood of finding and keeping great stones, but all things considered, this pull request seems like the easier option.

Additional context
I created a pull request that makes the magic happen:
#135
(i'd even prefer to make this option required, but did not do it to avoid breaking stuff)