Added filter functionality

[alejandroliu]

This PR introduces "filter" functionality. It similar to the tool feature, but it will take the tool output
and embed it into the output documentation file in-place.

Example use:

/****m* tools/filtertest
 *
 * NAME
 *   FilterTest
 *
 * DESCRIPTION
 *   Example showing how to invoke external filter
 *
 * EXAMPLE
 * This one formats markdown input.  This requires a `markdown` command to exist:
 * The input data is passed through stdin.
 *
 * |filter markdown
 * This is markdown text:
 *
 * Use like *this* or __that__.
 * |filter end
*
* Using pandoc:
* 
* |filter pandoc -f csv -t %output_mode%
 * Country, Sales
 * Netherlands, 100
 * Belgium, 500
 * Luxemburg, 50
 * Total, 650
 * |filter end
 *
 ******
 */

To support multiple output formats, the %output_mode% is replaced to one of the following:

ascii, latex, html, rtf, troff, xmldocbook, raw

So the filter can then arrange the output accordingly. For example, a tool like pandoc
can be used to parse multiple mark-up formats and generate HTML, RTF, LaTeX, docbook, roff.

This means that a number of feature requests on the issues list could be implemented via external filter. For example:

• Jade pipe support (Feature Request) #10 : I don't know Jade, but I imagine this could be done via a jade filter.
• Multi-line HTML piping (Feature Request) #9 : This can be implemented trivially with using cat command as a filter.
• Markdown support (Feature Request) #8 : This could be done with pandoc or any user provided markdown converter