BTCMarkets: Add subscription configuration

[gbjk]

BTCMarkets

• Add subscription configuration
• Add subscription.List.GroupByPairs and subscription.List.Authenticated
• Group subscriptions by pairs to reduce subscription messages
• Rename default config exchange name to match package name
• Subscribe to heartbeat when not authenticated

Type of change

• New feature (non-breaking change which adds functionality)

[codecov]

Codecov Report

Attention: Patch coverage is 62.22222% with 17 lines in your changes missing coverage. Please review.

Project coverage is 38.27%. Comparing base (17c2ef2) to head (a759760).
Report is 2 commits behind head on master.

Files	Patch %	Lines
exchanges/btcmarkets/btcmarkets_websocket.go	32.00%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1624      +/-   ##
==========================================
+ Coverage   36.37%   38.27%   +1.90%     
==========================================
  Files         422      422              
  Lines      183113   152325   -30788     
==========================================
- Hits        66602    58305    -8297     
+ Misses     108466    85976   -22490     
+ Partials     8045     8044       -1     

Files	Coverage Δ
exchanges/btcmarkets/btcmarkets_wrapper.go	36.39% <100.00%> (+3.62%)	⬆️
exchanges/subscription/list.go	100.00% <100.00%> (ø)
exchanges/subscription/subscription.go	100.00% <ø> (ø)
internal/testing/exchange/exchange.go	46.90% <100.00%> (+4.04%)	⬆️
exchanges/btcmarkets/btcmarkets_websocket.go	47.30% <32.00%> (+7.45%)	⬆️

... and 394 files with indirect coverage changes

[shazbert]

Nice work! Just one nit.

[shazbert]

ch ACK. Thanks.