New regex input plugin 

added regex input plugin. A more complete offering of what the text plugin currently does. Specifically support for files, flags, regexs with newlines, and detailed support for custom fields. 0 errors with <code>pep8 --max-line-length=100 --first regex_input.py</code>. I didn't want to send a request to replace the text plugin as my code uses a different config format and would break anyone's setup that uses it.

Example config parse ezrss.it rss feed:
```yml
regex-input:
  resource: http://username:password@ezrss.it/feed/
  title:
    - {regex: '(?<=<title><!\[CDATA\[).*?(?=\]\]></title>)'} #comment
  url:
    - {regex: "magnet:.*?(?=])"}
  sections:
    - {regex: "(?<=<item>).*?(?=</item>)", flags: "DOTALL,IGNORECASE"}

  custom-keys:
    custom_field_name:
      regexs:
        - {regex: "regex for first keep to keep", flags: "DOTALL"}
      required: False
    custom_field_name2:
      regexs:
        - {regex: "regex for a field to keep"}
```