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

doc_test.go

@@ -6,52 +6,6 @@ import (
 	"fmt"
 )
 
-func ExampleNodeFilterFn_filterExample() {
-	tree, _ := Load(`
-      [struct_one]
-      foo = "foo"
-      bar = "bar"
-
-      [struct_two]
-      baz = "baz"
-      gorf = "gorf"
-    `)
-
-	// create a query that references a user-defined-filter
-	query, _ := CompileQuery("$[?(bazOnly)]")
-
-	// define the filter, and assign it to the query
-	query.SetFilter("bazOnly", func(node interface{}) bool {
-		if tree, ok := node.(*TomlTree); ok {
-			return tree.Has("baz")
-		}
-		return false // reject all other node types
-	})
-
-	// results contain only the 'struct_two' TomlTree
-	query.Execute(tree)
-}
-
-func ExampleQuery_queryExample() {
-	config, _ := Load(`
-      [[book]]
-      title = "The Stand"
-      author = "Stephen King"
-      [[book]]
-      title = "For Whom the Bell Tolls"
-      author = "Ernest Hemmingway"
-      [[book]]
-      title = "Neuromancer"
-      author = "William Gibson"
-    `)
-
-	// find and print all the authors in the document
-	authors, _ := config.Query("$.book.author")
-	for _, name := range authors.Values() {
-		fmt.Println(name)
-	}
-}
-
 func Example_comprehensiveExample() {
 	config, err := LoadFile("config.toml")
 
@@ -71,11 +25,5 @@ func Example_comprehensiveExample() {
 		// show where elements are in the file
 		fmt.Printf("User position: %v\n", configTree.GetPosition("user"))
 		fmt.Printf("Password position: %v\n", configTree.GetPosition("password"))
-
-		// use a query to gather elements without walking the tree
-		results, _ := config.Query("$..[user,password]")
-		for ii, item := range results.Values() {
-			fmt.Printf("Query result %d: %v\n", ii, item)
-		}
 	}
 }