Add -E tag:name:value syntax to get a value from name/value pairs

[gstaples]

This is support name/value pairs like this:

-E tag:value:value is for extracting  name/value pairs.  Given:
   [{ tagname: color, tagvalue: red },
    { tagname: texture, tagvalue: squishy },
    { tagname: height, tagvalue: 10ft }]
   jshon -a -E tagname:height:tagvalue -> 10ft

It's not beautiful, but I'm happy to work with you on the syntax.