tbls diff support for diff checking between dsn and dsn

[k1LoW]

$ tbls diff postgres://dbuser:*****@local:5432/dbname postgres://dbuser:*****@production:5432/dbname

[codecov]

Codecov Report

Merging #280 (ca6f6c1) into master (d50ec0b) will decrease coverage by 0.75%.
The diff coverage is 69.44%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
- Coverage   66.21%   65.45%   -0.76%     
==========================================
  Files          24       24              
  Lines        4031     4099      +68     
==========================================
+ Hits         2669     2683      +14     
- Misses       1085     1137      +52     
- Partials      277      279       +2     

Impacted Files	Coverage Δ
output/md/md.go	68.77% <69.44%> (-0.19%)	⬇️
datasource/datasource.go	21.67% <0.00%> (-12.17%)	⬇️

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