support arbitrary keys

[milahu]

now users must append jsonMap.leaf_node to their keyList like

const jsonMap = require('json-source-map');
const result = jsonMap.parse('{"z":1,"a":{"z":1,"a":1}}');
const keyList = ['z', 'a']; // before this was ptr = '/z/a'
let pointer = result.pointers;
for (let key of keyList.concat([jsonMap.leaf_key])) {
  pointer = pointer[key];
}
const [a, b] = [pointer.value.pos, pointer.valueEnd.pos];

edit: forgot keyList = .... in commit message

edit: the constant should not be called leaf_key, rather loc_key

todo: tests ....

[coveralls]

Coverage decreased (-18.5%) to 81.495% when pulling 37dd775 on milahu:patch-1 into 3e58861 on epoberezkin:master.