Add support for scanning joined assoc

[Fs02]

Scan query result to embedded has one or belongs to association when the returned query result have field path to the association (example: "assoc_name.field_name")

example:

var users []Users
err := repo.FindAll(ctx, &users, rel.Select("*", "follower_stats.followers_count").Join("follower_stats").Where(where.Gt("follower_stats.followers_count", 100)))

related: #287

[codecov]

Codecov Report

Merging #290 (8858030) into master (115ffc0) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #290   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           33        33           
  Lines         2778      2802   +24     
=========================================
+ Hits          2778      2802   +24     

Impacted Files	Coverage Δ
document.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 115ffc0...8858030. Read the comment docs.

[lafriks]

how does it distinguish columns with same name but form different tables in select?

[Fs02]

@lafriks the field name have to be prefixed with table name, my above example is probably not valid, probably it should be "follower_stats.followers_count" as "follower_stats.followers_count"