Add ability to export group

This adds an option to export a group instead of the entire table, as requested in #84.

Before merging in, the UX should perhaps be considered. This adds two options to the _Export_ context submenu: _Export Group to CSV_ and _Export Group to HTML_. These only appear when grouping has been turned on. This works alright, but there's also a _Grouping_ submenu that could also hold these options. The place they appear now makes the most sense to me, but I can move them elsewhere if desired.

Code-wise, the way I tell `export` to only export a group may seem a bit strange. I pass it the row that the group is on then later call `getGroupFromRow` to get the underlying group. Initially I passed the group itself (also retrieved via `getGroupFromRow`) as an argument to `export`, which makes more sense, but when remotely fetching rows these will contain placeholders and you have to "re-get" the group anyway. Getting the group from the clicked row inside `export` is currently the most elegant way to do this.
`groupRow` is the row number that is clicked. It's used inside the `export` function to get the group object itself. I don't pass the group object because the column values are placeholders before a remote fetch. Passing the row number was the only straightforward way to retrieve the group inside `export` (see the description of #168).

Of course, I can rename the variable to be more descriptive. Also open to other suggestions for getting the group given the arguments passed to the context menu event handler.

I'll update the comments.