Fix ls --recursive discrpency with prefixes

[kyleknap]

Fixes #604

As an example of what is changed, start off with a bucket mybucket with the key foo/bar.

When we used to run aws s3 ls s3://mybucket/foo/ --recursive, it would list the object as foo/bar. This is inconsistent with aws s3 ls s3://mybucket/foo/ since it would list the object as bar.

This change makes the output consistent with the non-recusive ls.

cc @jamesls @danielgtaylor

[kyleknap]

Currently, this PR will be too big of a breaking change to merge in. So instead, I propose to add extra arguments to the ls command that will allow to control the format of the keys. Here are the two options I am considering:

1. Have a --relative and --absolute or something along those lines that will list the keys relative to the s3path provided or list the full key name respectively. This would affect both the recursive and non-recursive case.

2. Have a --key-format argument whose value is relative or absolute and do the same functionality as in option 1.

If anyone has any opinions on this, I would greatly appreciate it. Please feel free to share. I am leaning toward option 2 because it is a little more scalable if there are any future format that are needed, will avoid bloating the amount of arguments in ls, and ensure only one key format can be specified for an ls command.

[jamesls]

Closing for now. If anyone has any feedback, please feel free to chime in.