Add ability to limit base currency for market pairs

[keithfawcett]

I like not having to list all market pairs by leaving the "market_pairs" blank, but I would like to limit what is scaned to only market pairs with BTC as the base currency.

For example, I want to see ETH/BTC, XLM/BTC, and others, but not XLM/ETH, XLM/BNB, etc.

[ghost]

Could probably add some kind of wildcard filtering.