Expand default column / header options

[tmikoss]

Based on feedback in #332

• Expand default column options to allow setting a global header naming scheme
• Add default filter options

[tmikoss]

WIP, avaiting your feedback @bogdan

The main issue I ran into while implementing - default_column_options already exists, and it expects an object, instead of a proc. And making default_filter_options the other way around would not make sense.

I've started with converting header proc to receive column / filter definition in params. But I suppose to be truly useful, others should have that logic too.

Would it make sense to give the whole default_column_options the callable treatment, allow it to be either an object, or column -> object proc? And same with filter?

Edit: The more I think about it, the more it seems that the only way for this feature to be flexible enough is by passing column / filter instance to the proc. Otherwise there would be either too many changes to other options beside header, or questions of "well why not this one?" sort. Updated the implementation. If the idea itself looks solid, I'll go about updating docs / tests.

[bogdan]

Looks way better than I expected! Please add/update the doc on the new options.