Padding option

Dunno if this is useful, or indeed if it's already a feature and I've just ignored it!

Anyway, I've added a 'padding' option to the default config, which is then interpreted at the init() function, with any initial coordinates giving a set padding either side (so if we zoom in on 1:68903942 with padding set to '5', it will show the region 1:68903937-68903947)