Add open_revs support

[flimzy]

Fixes #398

[codecov]

Codecov Report

Patch coverage: 63.76% and project coverage change: -0.17% ⚠️

Comparison is base (102b906) 82.50% compared to head (8709230) 82.34%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
- Coverage   82.50%   82.34%   -0.17%     
==========================================
  Files          85       85              
  Lines        9009     9067      +58     
==========================================
+ Hits         7433     7466      +33     
- Misses       1368     1387      +19     
- Partials      208      214       +6     

Files Changed	Coverage Δ
couchdb/chttp/chttp.go	96.35% <33.33%> (-0.70%)	⬇️
couchdb/db.go	78.07% <65.15%> (-1.89%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.