feat: drop cascade & restrict

[aldy505]

New feature: DROP TABLE tableName RESTRICT; and DROP TABLE tableName CASCADE;.

Usage:

  sql, _, err = bob.DropTable("users").Cascade().ToSql()
  if err != nil {
    log.Fatal(err)
  }
  // sql = "DROP TABLE users CASCADE;"
  sql, _, err = bob.DropTable("users").Restrict().ToSql()
  if err != nil {
    log.Fatal(err)
  }
  // sql = "DROP TABLE users RESTRICT;"

Closes #5

[codecov-commenter]

Codecov Report

Merging #7 (8a461fe) into master (efa752f) will decrease coverage by 2.13%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##            master       #7      +/-   ##
===========================================
- Coverage   100.00%   97.86%   -2.14%     
===========================================
  Files            8        9       +1     
  Lines          263      281      +18     
===========================================
+ Hits           263      275      +12     
- Misses           0        6       +6     

Impacted Files	Coverage Δ
internal.go	45.45% <45.45%> (ø)
drop.go	100.00% <100.00%> (ø)
has.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 efa752f...8a461fe. Read the comment docs.