Crash on negative list index - could traverse backwards instead

[krangerich]

Describe the bug
When you specify a negative list index, for example "yr r data1.yaml foo[-1]", the program crashes with a stack trace

Input Yaml
data1.yml:

foo:
- bar: baz

Command
The command you ran:

yq r data1.yaml foo[-1]

Actual behavior

runtime error with stack trace

Expected behavior

Print an error message
OR
implement new feature like "-1 means last item of the list"

[mikefarah]

Ooh I like negative indexes for going backwards, that would be cool.