MPD protocol: make it more robust
MPD protocol: make it more robust

 * play command: no args or -1 does "unpause"
 * random command: noop instead of error
 * single command: fix the logic to accept the argument
 * optional ranges allow -1 to mean no argument

emersion/groovebasin,gorlug/groovebasin,andrewrk/groovebasin,CalebMorris/groovebasin,sammcj/groovebasin,sammcj/groovebasin,gorlug/groovebasin,CalebMorris/groovebasin,andrewrk/groovebasin,andrewrk/groovebasin,melissanoelle/groovebasin,CalebMorris/groovebasin,gorlug/groovebasin,dwrensha/groovebasin,neynah/groovebasin,emersion/groovebasin,neynah/groovebasin,Qbuer/groovebasin,melissanoelle/groovebasin,emersion/groovebasin,dwrensha/groovebasin,dwrensha/groovebasin,Qbuer/groovebasin,dwrensha/groovebasin,Qbuer/groovebasin,neynah/groovebasin,sammcj/groovebasin,melissanoelle/groovebasin