Couple of fixes/enhancements to carbon-path

See commit list :)

tl;dr examples:

```
» carbon-list | head -n2 | carbon-path -p
/opt/graphite/storage/whisper/collector_time_ms.wsp
/opt/graphite/storage/whisper/mapr/cluster/collector_time_ms.wsp

» carbon-list | head -n2 | carbon-path -p | carbon-path -r
collector_time_ms
mapr.cluster.collector_time_ms

» carbon-list | head -n2 | carbon-path -p -d whatever | carbon-path -r
whatever.collector_time_ms
whatever.mapr.cluster.collector_time_ms

» carbon-list | head -n2 | carbon-path -p -d whatever | carbon-path -r -d whatever
collector_time_ms
mapr.cluster.collector_time_ms

» carbon-list | head -n2 | carbon-path -p -d /whatever/ | carbon-path -r -d whatever
collector_time_ms
mapr.cluster.collector_time_ms
```