Add Map transform

[eliascarv]

The Map transform applies a function to table columns using the map function.
The following codes are equivalent:

table |> Map([:a, :b] => ((a, b) -> 2a + b) => :c)

table.c = map((a, b) -> 2a + b, table.a, table.b)

[codecov-commenter]

Codecov Report

Merging #187 (1aa9dca) into master (9aeb688) will decrease coverage by 4.92%.
The diff coverage is 0.00%.

❗ Current head 1aa9dca differs from pull request most recent head 69444cb. Consider uploading reports for the commit 69444cb to get more accurate results

@@            Coverage Diff             @@
##           master     #187      +/-   ##
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- Coverage   91.76%   86.85%   -4.92%     
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  Files          30       31       +1     
  Lines        1008     1065      +57     
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  Hits          925      925              
- Misses         83      140      +57     

Files Changed	Coverage Δ
src/TableTransforms.jl	100.00% <ø> (ø)
src/transforms.jl	98.61% <ø> (ø)
src/transforms/map.jl	0.00% <0.00%> (ø)