Add support for Proc/lambda on column_types

[westonganger]

It would be good to support Proc/lambda for column types

This could be supported as

.to_xlsx(
  column_types: [
    :string, 
    ->(cell_val){ cell_val.start_with?("http") ? :hyperlink : :string }, 
    :float,
  ]
)

We dont need to do anything additional for spreadsheet_columns because its already instance evaluated so you can already do things like

.to_xlsx(
  spreadsheet_columns: ->(){
    [
      "Foo", :foo, :string
      "Bar", :bar, (bar.start_with?("http") ? :hyperlink : :string)
      "Baz", :baz, :float
    ]
  }
)

[westonganger]

This is being implemented in #51 if you want to try this out

[westonganger]

This is now implemented in master.