Input data format: value

[sparrc]

we should be able to parse simple values as a data format, with the ability to specify string, integer, float or boolean, like this:

[[inputs.exec]]
  commands = ["uname -r"]
  data_format = "value"
  data_type = "string"
  name_override = "kernel_version"

which would output:

kernel_version,[tags]  value="15.3.0"  1457975011144006845

see #235