[polyglot-yaml] Add ability to customise list element types

[Tzrlk]

Following on from #151, there should be an annotation that allows for list items to be customised in special circumstances. Given the following configuration:

configuration:
  pluginOptions:
    - option1
    - option2

The xml that gets generated ends up like this:

<configuration>
  <pluginOptions>
    <pluginOption>option1</pluginOption>
    <pluginOption>option2</pluginOption>
  </pluginOptions>
</configuration>

Unfortunately, in my specific case, the xml generated needs to be like this:

<configuration>
  <pluginOptions>
    <option>option1</option>
    <option>option2</option>
  <pluginOptions>
</configuration>

A potential solution is to add a tag option (http://yaml.org/spec/1.1/#id858961) which gets read and interpreted as the element type (examples here), e.g.:

configuration:
  pluginOptions: !option
    - option1
    - option2

or add parameterisation to the tag and try to find the metadata during parsing:

configuration:
  pluginOptions: !xml:element:option
    - option1
    - option2

If tags aren't usable, the parser could always look at lists and split the name on :. e.g.:

configuration:
  pluginOptions:option:
    - option1
    - option2

Or tags could be used statically to specify an element with metadata:

configuration:
  pluginOptions: !xml:list
    elementType: option
    elements:
      - option1
      - option2

[Tzrlk]

Option 2 is my favourite, but option 4 is a lot easier to implement. Make sure you have a class to hold the element name in, and implement things like in the snakeyaml examples.

[SanderSmee]

I read through the issue list and think #44 or #122 can help you out.