Reticker

* Allow mapping a ticker to multiple tickers (and not merely to only a single ticker as before). Refer to the example in the readme.

* Change an example in the readme.

* Support a configurable mapping via `reticker.config.MAPPING` of one ticker to another, e.g. BTC to BTC-USD. Refer to the readme for usage.
* [breaking] `reticker.BLACKLIST` is no longer available. It is now available only as `reticker.config.BLACKLIST`. Refer to the readme for usage.

* Improve readme.

* Allow optionally matching `prefixed_uppercase`, e.g. $ARKX. This had been enabled by default, but can now optionally be disabled.
* Allow optionally matching two-part tickers, e.g. BRK.A, BRK-B, MGC=F. This is now enabled by default, but can optionally be customized or disabled by specifying `separators`.

* Add terms to blacklists.