feat: added some alter function in addition to drop builder

[aldy505]

Closes #8

[codecov-commenter]

Codecov Report

Merging #14 (1f53963) into master (0e71945) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #14   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        11    +1     
  Lines          335       373   +38     
=========================================
+ Hits           335       373   +38     

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

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