Allow multimple selects

[fairking]

This change would allow for multiple selects.

For example:

var query = rel.Select("id", "title").From("pools")

if includeAvgScore {
    query.Select("avg_score AS score")
} else {
    query.Select("score")
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7ac79bf) to head (e9d4756).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #369    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           35        35            
  Lines         4085      3462   -623     
==========================================
- Hits          4085      3462   -623     

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