Add support for pulling text from node

Author: nobodywasishere

src/tree_sitter/capture.cr

@@ -13,6 +13,10 @@ module TreeSitter
       node.to_s(io)
     end
 
+    def text(source : String) : String
+      node.text(source)
+    end
+
     def inspect(io : IO)
       io << "#<Capture "
       to_s(io)

src/tree_sitter/node.cr

@@ -122,6 +122,13 @@ module TreeSitter
       io.write(bytes)
     end
 
+    def text(source : String) : String
+      start_pos = start_byte
+      end_pos = end_byte
+      slice = source.byte_slice(start_pos, end_pos - start_pos)
+      @@string_pool.get(slice)
+    end
+
     # :nodoc:
     def to_unsafe
       @node