Add support for Editor Coroutines

[jdnichollsc]

Use EditorCoroutine option to support Editor Coroutines for a request

[maifeeulasad]

@jdnichollsc, taking a look at it.

[jdnichollsc]

To avoid this dependency maybe we can have another branch for this feature, what do you think?

[maifeeulasad]

It's awesome.

Just on a side note:

• Maybe we can add some examples, it may come in handy (I know it's more like, how to write an Editor scripts and all)

[RunoLight]

To avoid this dependency maybe we can have another branch for this feature, what do you think?

Maintaining multiple versions (with and without editor coroutines) could get quite complex. I think it's possible to support both cases using a conditional compilation flag — something like #if with a specific preprocessor define. Odin Serializer and Inspector use a similar approach with certain libraries. They then just notice about that in readme, so user will know he can use that and how to enable it.