added support for 'replace_dot_with' flag in ES encoders

[jxstanford]

This PR adds a new replace_dots_with parameter to ESJsonEncoder and ESLogstashV0Encoder. The parameter value is a string that will be substituted for all . characters in all keys of the unmarshalled JSON value found in the message Payload. Tests have been updated accordingly.

Here is an example encoder config:

[ESJsonEncoder]
index = "gke-%{Logger}-%{%Y.%m.%d}"
es_index_from_timestamp = false
type_name = "%{Type}"
replace_dots_with = "_"

Closes: #1946

[rafrombrc]

Not sure what the value of replaceDotsWith function is over the stdlib's strings.Replace:

strings.Replace(f.GetName(), ".", replaceDotsWith, -1)

[rafrombrc]

Thanks for your contribution. In addition to the inline comment I made, you'll want to update the documentation (see esjson.rst and eslogstashv0.rst files in the docs/source/config/encoders folder) and add an entry to the CHANGES.txt changelog.

[jxstanford]

@rafrombrc no problem. really excited about working with Heka. all comments addressed...

[rafrombrc]

Great, thanks!