Allow regexp processor to mix different tags

[adrianlzt]

This change allows to mix values from different tags.
When regexp is used to transform a tag, if value append is set to
true and we the result_key tag already exists, replacement value
will be appended to the end of the current tag value.

Example use case, set host tag with the values of tags clustername and dcname:
Input:

vsphere_cluster_vmop,clustername=somecluster,dcname=mydc,host=xxxx numPoweron_latest=27i 1557967800000000000

Processor config:

[[processors.regex]]
  [[processors.regex.tags]]
    key = "clustername"
    pattern = "^(.*)"
    replacement = "${1}"
    result_key = "host"
  [[processors.regex.tags]]
    key = "dcname"
    pattern = "^(.*)"
    replacement = "_${1}"
    result_key = "host"
    append = true

Output:

vsphere_cluster_vmop,clustername=somecluster,dcname=mydc,host=somecluster_mydc numPoweron_latest=27i 1557967800000000000

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.