feat: add ability to initially sort a column descending

[mayorbyrne]

After playing around with gridJS I noticed there wasn't a way to have columns sort descending on first click. Normally with numbers, this is the way you want sorting to behave (highest numbers first, to lowest). With this addition you can set the direction of the column sort in config.

      columns: ['Name', 'Email', 'Phone Number', {
        name: 'Age',
        sort: {
          direction: -1,
        }}]

[uwemaurer]

Good feature, I need this too.

@afshinm how about merging this feature?