[FeatureRequest] Summary: data from Analyzers.

Subj:)

Real-world scenario: I have some values that are determined during analysis pass and I want to include them into summary table. Current implementation initializes the table immediately after benchmark run so there's no way to pass the values from the analyzer.

So far there are two options I thought about:

* Do not init the table until all analysers are complete. Will break all analyzers that use `summary.Table`. Not a bad thing though, as the table contains only text values and the code has to parse them to obtain numbers. It shall fail anyway, so the change is not so breaking:)

* Or, add `GetColumnProvider()` to the `IAnalyzer` and add columns from it on analysis pass completion. It will make code much more complex as you'll had to insert/replace columns (if there are old ones with same Ids) dynamically.

NB I'll be satisfied with any another solution, these two are just an example.