Add MongoURIStore

[shyamd]

Adds a MongoURI Store for special mongodb+srv:// URIs that fully setup the mongo client from DNS records

[codecov]

Codecov Report

Merging #93 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #93      +/-   ##
=========================================
+ Coverage   86.97%   87.1%   +0.13%     
=========================================
  Files          20      20              
  Lines        1443    1458      +15     
=========================================
+ Hits         1255    1270      +15     
  Misses        188     188

Impacted Files	Coverage Δ
src/maggma/stores/mongolike.py	94.21% <100%> (+0.49%)	⬆️
src/maggma/stores/__init__.py	100% <100%> (ø)	⬆️

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[lgtm-com]

This pull request introduces 1 alert when merging 5e27ddd into 98642b8 - view on LGTM.com

new alerts:

• 1 for First argument to super() is not enclosing class