Capability to set / remove comment lines, parse mode normal / numeric only / raw

[rik72]

This is my attempt at adding the following additional capabilities:

• set / remove comment lines
• set parsing mode
  • PR_NORMAL: the original behaviour, parsing "on"/"true"/"yes" to PHP true, etc
  • PR_NUMERIC: parsing to integer / float values only numeric values
  • PR_RAW: setting all values to string format with quotes

I changed just one single behaviour: adding a parameter to a section will put it before blank lines. This is to consistently use blank lines as separator between sections, which is in my opinion desirable for readability.