Powerful querying interface

As pointed out by @eanderton in https://github.com/pelletier/go-toml/pull/21#issuecomment-53159667, it would be great to implement a powerful way to query the tree structure.

Possible ideas:

* [XPath](http://en.wikipedia.org/wiki/XPath)
* [Bash-like pattern matching](https://www.gnu.org/software/bash/manual/html_node/Pattern-Matching.html)
* Custom DSL

Thoughts, ideas and suggestions are welcome! 
Good catch.  That's an oversight - I'm in the middle of documenting all this, so I'll change that before the next commit.