Add padding option to defaultBounds

[francescocretti]

As discussed in issue #391, this PR adds the optional padding property to defaultBounds.

e.g.

    <Map
      defaultBounds={{
        west: -7.521054546833144,
        north: 57.18946776545227,
        south: 49.60181300450985,
        east: -2.8939254862738877,
        padding: {
          bottom: 50,
          right: 50,
        }
      }}
    />

NOTE
The Google Maps API implement a system of discreet zoom levels, so some values of padding could not be visible if they don't reach the smallest amount of padding necessary to cause the zoom level to change by a full integer value. More info: https://stackoverflow.com/a/44476181

[usefulthink]

That was fast, thanks a lot! I'll merge that later today for the 1.1.0 version, I still have a fix for 1.0.2 to do before that..

[francescocretti]

@usefulthink no problem ;) thanks for your work. Let me know if changes are needed!