Suggestion for template: sticky header

Hi!
The header of the table contents can become sticky when scrolling, if you use:
```
#files.tablesorter thead tr th {
	position: sticky;
	top: 0;
}
```
Maybe you can consider putting it in the template.
Support for `position: sticky` can be seen here: https://caniuse.com/css-sticky

Thanks for a great software!