Bitstamp: Add subscription configuration

[gbjk]

Type of change

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

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 20 lines in your changes missing coverage. Please review.

Project coverage is 38.26%. Comparing base (17c2ef2) to head (7593a5f).
Report is 7 commits behind head on master.

Files	Patch %	Lines
exchanges/bitstamp/bitstamp_websocket.go	28.57%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1620      +/-   ##
==========================================
+ Coverage   36.37%   38.26%   +1.89%     
==========================================
  Files         422      422              
  Lines      183113   152302   -30811     
==========================================
- Hits        66602    58277    -8325     
+ Misses     108466    85978   -22488     
- Partials     8045     8047       +2     

Files	Coverage Δ
exchanges/bitstamp/bitstamp_wrapper.go	59.27% <100.00%> (+6.03%)	⬆️
exchanges/subscription/list.go	100.00% <100.00%> (ø)
exchanges/subscription/subscription.go	100.00% <ø> (ø)
exchanges/bitstamp/bitstamp_websocket.go	38.93% <28.57%> (+7.13%)	⬆️

... and 396 files with indirect coverage changes

[shazbert]

tACK. Nice :D

[gbjk]

Switched Authenticated for Public, merging #1629