Move query to its own subpackage (#152)

Move all the query system to its own package. The reason is to
avoid it to rely on unexported methods and structures, and move
it out of the main package since this is really not a core
feature. It is still tied to the toml.TomlTree and toml.Position
structures for now.

* Move query mechanism to its own subpackage
* Rename QueryResult to Result to avoid stutter
* Add query.CompileAndExecute

Fixes #116

README.md

@@ -18,7 +18,7 @@ Go-toml provides the following features for using data parsed from TOML document
 * Load TOML documents from files and string data
 * Easily navigate TOML structure using TomlTree
 * Line & column position data for all parsed elements
-* Query support similar to JSON-Path
+* [Query support similar to JSON-Path](query/)
 * Syntax errors contain line and column numbers
 
 Go-toml is designed to help cover use-cases not covered by reflection-based TOML parsing: