Added pre submit code modifiers

[veryshyjelly]

added modifiers for modifying code just before submission

example usage:

  kotlin:
    out_dir: src
    post-modifiers:
      - script: |
          function modify(code) {
             return `class Solution {\n    ${code.split('\n').join('\n    ')}\n}`;
          }
    modifiers:
      - name: removeUselessComments
      - script: |
          function modify(code) {
            const functionBodyMatch = code.match(/class Solution \{([\s\S]+)\}/);
            if (functionBodyMatch) {
              return functionBodyMatch[1].trim(); 
            }
            return '';          
          }

[j178]

Can we integrate the pre-submit modification directly into the lang.GetSolutionCode function? What do you think?

[j178]

It might be clearer to name it something like pre-submit-modifiers.

[j178]

Thanks for your contribution!

Could you also update the README.md to mention this change?