Parser/processor for simple tags

Re-rolled the patch for the latest 2.x

I've tested this with a simple blockquote paragraph. It works as expected:

```
field_content:
plugin: html_to_paragraphs
source: body
parser:
-
plugin: html_parser_plain
tags:
- blockquote
process:
-
plugin: html_process_plain
tag: blockquote
bundle: quote
field_name: field_copy_basic
text_format: basic_rich_text
-
plugin: html_process_text
bundle: rich_text
field_name: field_rich_text
text_format: rich_text
fallback: true
```

This works for me, just what I needed :D

field_components:
    plugin: html_to_paragraphs
    source: content
    parser:
      -
        plugin: html_parser_plain
        tags:
          - blockquote
    process:
      -
        plugin: html_process_plain
        tag: blockquote
        bundle: quote
        field_name: field_blockquote
        text_format: full_html
      -
        plugin: html_process_text
        bundle: text
        field_name: field_text
        text_format: full_html
        fallback: true

Reviewed and tested.