Autogenerated documentation

**Suggestion**:

> update the documentation generator to support additional meta markup of configuration options

**Preamble**:
When trying out a new filter/input/something, the first thing one often do is prototype a POC to get a better understanding of how it works. A good way to do this is look at the documentation and check the required configuration options.

**Trouble**:
Sometimes the required configuration options varies based on other configuration options. This is communicated in the invidual option sections. Therefore you would have to read every configuration option to make sure that you have the required options.

**Example**:
[Logstash-filter-xml 5.0.0-alpha4](https://www.elastic.co/guide/en/logstash/5.0/plugins-filters-xml.html) documentation reports the required configuration options as

> `xml {
> source => ...
> }`

but the option "target" have the followin line

> Required if store_xml is true (which is the default).

meaning that the required configuration option actually is either option A

> `xml {
> source => ...,
> target => ...
> }`

or option B

> `xml {
> source => ...,
> store_xml => false
> }`

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[Discuss.elastic thread](https://discuss.elastic.co/t/target-required-xml-filter-option/54700)