Add base URL support

[lo5]

Launch waved with -base-url to set a base URL. Or, launch waved with $H2O_WAVE_BASE_URL set.

Setting base url to /foo/ will allow serving an app with route / at /foo/, or an app with route /bar at /foo/bar/.

Accessing /foo/'s parent(s) from browser/API will 404.

If a base url is set, an app must be launched with $H2O_WAVE_BASE_URL set to the same base url value.

Closes #59