Add domain specification

[manafire]

Allow users to specify domains in addition to IPs. See: #306

[szemek]

@manafire recently I run into issue that my web server binds to Unix domain socket and code fails when executing allow_ip?(env).

What do you think about changing order of the execution

if allow_domain?(env) || allow_ip?(env)

?

In that case I could solve my problem with:

BetterErrors::Middleware.allow_domain!('socket')

[manafire]

@szemek I have changed the order as you've requested. Hope that works for you.

[rubemlrm]

its possible merge this pull request ? =) i'm working with vagrant env and i'm using custom domain for that instead ips .

[RobinDaugherty]

@rubemlrm using a custom domain is not secure, as was discussed in #306. Please see the wiki for instructions on setting up vagrant correctly.

[rubemlrm]

@RobinDaugherty sorry for the duplicated error :/ i will check that later , and i will test with docker too , since i've discard the vagrant usage to use docker instead. But anyway thanks for the reply and support =)