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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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