Replace blocking client with lighter one

[nicomem]

(Based on the branch at #11, this should not be merged before the base branch)

This PR makes those changes:

• Replace the reqwest blocking client with ureq, which is a lot lighter (~100 crates less)
• Add the async feature, for users to be able to remove the async deps when using the blocking client
  • The async feature is added in the default features, so that it isn't a breaking change
• Use different structs for the blocking and async features
  • This is necessary to avoid the "multiple implementation" compile error
  • To avoid a breaking change, export YahooConnector as either:
  • The async version if the blocking feature is not present
  • The blocking version if the async feature is present

[xemwebe]

Though I understand the demand for a lighter http client, I would prefer to have this implemented as an alternative, not as a replacement. If the library is used in context where reqwest is used anyway, which is not unlikely, since it is the most widely used http client crate, switching to another client would indeed increase the dependency burden instead of reducing it. In addition, features like compilation to WASM targets does not seem to be supported by ureq (or at least that would have to be checked first). Therefore, I am a bit reluctant to accept this PR as it is.