Enhance memberlist csv column filtering

[mathieuhays]

Implements #481.

I've changed the way the columns array works. I've added the heading as the key (as you mentioned @strangerstudios) and I also added the ability to specify a callback which enabled me to include columns like "expires" or "joined" as filterable columns.

$extra_columns are merged with $default_columns since the behaviour is now similar.

pmpro_members_list_csv_heading still works but will break if someone tries to add/remove columns using this filter.

I've done some tests and we can now remove and/or re-order columns using pmpro_members_list_csv_default_columns. Actually it's now easier to remove/re-order columns because we are using the heading as the key.

I haven't managed to test the use-case for "oldmembers".

[strangerstudios]

This is super cool, but won't make the 1.9.4 release. I do want to take a deeper look at this before the next release for potential inclusion. I hope it's not too much trouble for you to repatch your site if you are using this already.

[sc0ttkclark]

We'll need to update the branch on mathieuhays:csv-header-enhancement with the latest from our current dev branch to move forward here. The changes in the PR look pretty good, I'd only suggest a few tweaks for new hooks (hook docs info which I can help with after PR is refreshed) and possibly a couple of code formatting things.