allow option to present context in reading output table

[edaniszewski]

when reading from a device which may have multiple readings, e.g.

$ ./synse plugin read 17074ac0-f08f-5c6a-96ed-209b1d16a067 
ID                                     VALUE                 UNIT   TYPE          TIMESTAMP
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   false                        bool          2020-05-20T14:58:58Z
17074ac0-f08f-5c6a-96ed-209b1d16a067   true                         bool          2020-05-20T14:58:58Z

it isn't possible to tell which reading is what in the table view. outputting to json or yaml gives this information, but it isn't as compact as the table view.

It would be nice to, either as a default or enabled by a flag, allow an additional "CONTEXT" column which would hold the context values for the reading.

It may take a little playing around to get this right and output in a useful way, since a reading may have many context fields.