extensibility with custom comparator functions

[jrencz]

I started working on next release which will incorporate new way to configure the filter: comparators.
Instead of restricting string comparison (which is default in 0.1) user will be able to compare other types with ability to implement his business logic into the filter itself where needed.
It may be found on https://github.com/EE/angular-ui-tree-filter/tree/wip-comparators
So far I provided proof-of-concept implementation for simple operator expressions like "> 10" or "<=15".

I'd appreciate any suggestions
cc
@pathes @mzgol @sicarrots @JimLiu @QcRafal

[jrencz]

corresponding highlight filter should be added to reflect matching logic. ui-utils one will no longer be suitable even for cases like simple operator expressions mentioned earlier.