Check if given email is included in black listed emails

Examples

iex> DisposableEmailBlacklist.valid?("my_email@gmail.com")
true

iex> DisposableEmailBlacklist.valid?("my_email@mailinator.com")
false

iex> custom_blacklist = ["hammermail.tk", "blastmail.org"]
iex> DisposableEmailBlacklist.valid?("my_email@hammermail.tk", custom_blacklist)
false