Allow relative width for the country dropdown list

[WuggyX2]

Hello,

I made these changes to allow developers to override the dropdown's width with relative lengths like %. Default lengths remain 300px

For example

<div>
  <PhoneInput
    inputStyle={{ width: "100%" }}
    dropdownStyle={{ width: "100%" }}
  />
</div>

I'm not sure if moving the country list outside the flag-dropdown div does break any featrue i'm unaware of.

fix #308