Add query variable support

[haohanyang]

Fixing #16

1. Added Query Variable Support. Each field name is a variable that has array value. Only string or number fields are supported. Format rules: https://grafana.com/docs/grafana/latest/dashboards/variables/variable-syntax/

The variable text has format <fieldname>:<value>
If a field has 1 element, then the element value is the variable value. Otherwise the variable value is the array

2. Changed default query to

[
  {
    "$limit": 10
  }
]