Improved handling of numeric keys

When there is no match at a given path, numeric keys are assumed to be strings.
To create an array '+' must be used.

e.g: yq n thing[+].cat fred
will create an array under thing, whereas
yq n thing[0].cat fred
will create a map under thing, with a key '0'

examples/instruction_sample.yaml

@@ -1,2 +1,2 @@
 b.c: cat
-b.e[0].name: Mike Farah
+b.e[+].name: Mike Farah