Add posfilter processor

[albfan]

Except for sparse searches, many times a search has an underliying pattern.

e.g. When searching "Foo" relating "function Foo" you may want to include only matches with "function" (search function definition) or exclude them (search function calls)

[albfan]

This feature aim to a plugin feature for ag.vim. A supposed implementation for definitions/callings:

• based on filetype, define a pattern to search callers and callees
• Activate postfilter
• Activate option (definition or callings)
• Do searchs
• Ease regular basis operation (Using unite, or some shortcut or custom functions)

[amerlyq]

Related to #36 and #34. Maybe split-recombine all those ideas will help to refine interface.

[albfan]

Use g:ag to define postprocesor variable. It will be a function which process each line and returns the filtered result.

e.g: pseudocode Implementation for definition/callings on vimL:

• Add a handler to AgSet or AgToggle
• Detect enable or disable to g:ag.callers or g:ag.callees with third party plugin which adds a postprocessor (can be only one var but this way seems more clear)
• postprocesor function filters lines with fu.*<search> at start (for definition), and not start with fu for callings

[albfan]

With 5f3ea8d place to put this is mostly defined.

Instead of an IgnorePattern, this match patterns will work finding callers/callees or similar