[sfputil]: Refactor and add support for presence, low-power mode and …

…reset (sonic-net#90)

- Convert sfputil to 'Click'-based command line utility
- Refactor sfputilbase.py: Clean up API and convert to proper abstract base class to make implementation more straightforward
- Add support for querying SFP transceiver presence
- Add support for querying & setting low-power mode
- Add support for resetting SFP transceivers

data/etc/bash_completion.d/sfputil

@@ -0,0 +1,8 @@
+_sfputil_completion() {
+    COMPREPLY=( $( env COMP_WORDS="${COMP_WORDS[*]}" \
+                   COMP_CWORD=$COMP_CWORD \
+                   _SFPUTIL_COMPLETE=complete $1 ) )
+    return 0
+}
+
+complete -F _sfputil_completion -o default sfputil;