Make the config environment variable override mechanism support environment variables that contain true/on or false/off.

As of today all environment variables set for overriding the json config will be reset to `true` or `false` if the value of the environment variables contain the string `true`/`on` or `false`/`off` respectively.

That means e.g.

`JSBIN_STORE_SQLITE_LOCATION=a/long/path`

results in a variable

`JSBIN_STORE_SQLITE_LOCATION=true`

since `long` contains **on**

This fixes the attempt to support leading and trailing spaces for the truthy and falsy values.