[RFC] Add (un)minification of css/js via ?debug= flag

DebugMode (serving unminified files) turned on by `?debug=true` (or other "truthy") value.  All other values equate to false (or `null`, see below).

**Acceptable "truthy" values:**
`yes, true, 1, on`

**Remove session value:**
`?debug=null`

---

**Examples**
````
http://localhost:8000/?debug=yes
http://localhost:8000/?debug=no
````

Addresses #21