allow to define theme colors and modify them

[rafalBujok]

It is an extended PR, it also contains #1270

themeColors array is 12 hex code strings in power point order
example of use:

    let newPptx = new pptxgen();
    newPptx.theme = {
      headFontFace: 'Helvetica Neue',
      bodyFontFace: 'Helvetica Neue',
      themeColors: ['000000', '000000', '000000', '000000', '000000', '000000', '000000', '000000', '000000', '000000', '000000', '000000'],
    };

fixing an issue #1243 created by @prescience-data but simplier implementation
also propobly fix for issue #604