Rename kivik.Options to kivik.Params, and make it a functional option

[flimzy]

No description provided.

[codecov]

Codecov Report

Patch coverage: 86.02% and project coverage change: +0.03% 🎉

Comparison is base (891e327) 82.93% compared to head (e7b707f) 82.96%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #690      +/-   ##
==========================================
+ Coverage   82.93%   82.96%   +0.03%     
==========================================
  Files          85       85              
  Lines        9022     9031       +9     
==========================================
+ Hits         7482     7493      +11     
+ Misses       1329     1328       -1     
+ Partials      211      210       -1     

Files Changed	Coverage Δ
mockdb/gen/parse.go	82.69% <ø> (ø)
mockdb/dbexpectations_gen.go	65.95% <59.09%> (ø)
mockdb/clientexpectations_gen.go	61.26% <62.50%> (ø)
options.go	38.46% <87.50%> (+11.18%)	⬆️
couchdb/replication.go	94.41% <100.00%> (ø)
couchdb/scheduler.go	94.65% <100.00%> (ø)
db.go	94.16% <100.00%> (ø)
mockdb/client.go	90.47% <100.00%> (ø)
mockdb/client_gen.go	80.21% <100.00%> (ø)
mockdb/common.go	77.41% <100.00%> (-2.59%)	⬇️
... and 5 more

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