Configurable popups so we don't show too many items

Reported by @kkarra: right now it looks like we show _all_ attributes of a class which isn't desirable behaviour if there are a lot of attributes. if the class for a given cell has 40+ attributes, all 40 show in the popover. We should limit this to summary fields and/or make it configurable.

Screenshot from YeastMine:

Some investigation notes:

At first I thought this might be a problem on the InterMine side because the same thing happened in BluegGenes, where tables look similar but have a totally different codebase.... but I managed to fix the bug in bluegenes, which quite clearly *was* iterating through all the attributes. I think im-tables js is doing the same, but I haven't been able to spot exactly _where_ yet. It _might_ even be configurable (see the [imtables configuring notes](https://github.com/intermine/im-tables/blob/master/CONFIGURING.md) and [options.coffee](https://github.com/intermine/im-tables/blob/9b06e2892b36226f4ecb5d7dce7cfc9422d087ac/src/options.coffee#L37) - but as noted in #92, not all options are clearly documented :(