Modify Frog to specify src and dst dirs

Hi Greg!

I'm using Frog to power my personal blog ([url](http://www.morepaul.com), [repo](https://github.com/paul-meier/Pablog)), and wanted to tweak my blog's build process to have easy cleans (i.e. 'rm -rf build'). I made some tweaks to my local Frog, and thought I'd offer them upstream if you were interested. Naturally, I'm happy to tweak my changes as you see fit.

Hope you're well! Below is the commit message:

---

As-is, Frog _must_ look for a top-level directory named `_src`, and _must_
put its generated output at the top-level. This means many of your site's
sources (js, css, favicon, etc.) must sit at the toplevel with your generated
output.

This change makes the source and destinations into parameters that can
be set from the config file. While the default values are the same
(`_src` and toplevel, for input and output, respectively), you can now
specify a directory for Frog's input and output, if you'd like to rename
"_src," or have the generated files go anywhere but the toplevel. See
`src-dir` and `output-dir` in `.frogrc` for more.