add String Converter to Daru::IO::CSV::CONVERTERS

[takkanm]

I added String Converter to Daru::IO::CSV::CONVERTERS.

I plan to apply this use case to columns that we do not want to be numbers.

irb(main):007:0> puts File.read('./example.csv')
id,"customer number",value
1,01,100
2,02,300
=> nil
irb(main):008:0> Daru::DataFrame.from_csv('./example.csv')
=> #<Daru::DataFrame(2x3)>
                    id customer n      value
          0          1          1        100
          1          2          2        300
irb(main):009:0> Daru::IO.from_csv('./example.csv', converters: [:string])
=> #<Daru::DataFrame(2x3)>
                    id customer n      value
          0          1         01        100
          1          2         02        300