Support changing site path with a environment variable SITE_ROOT.

[tetsurom]

Using reverse proxy I really want to change site path and bit disappointed with no progress for #6. So I have maid this request 😃

I tested these cases:

• without SITE_ROOT
• SITE_ROOT = /
• SITE_ROOT = foobar
• SITE_ROOT = /foobar
• SITE_ROOT = foobar/
• SITE_ROOT = /foo/bar/

[ikatson]

Thanks @tetsurom !