Handle multi-line meta in markdown content properly

Pelican should handle multi-line meta in markdown content properly.
It was handling first line only (except summary).

Here is corresponding document:
http://pythonhosted.org/Markdown/extensions/meta_data.html#syntax

> If a line is indented by 4 or more spaces, that line is assumed to be an additional line of the value for the previous keyword. A keyword may have as many lines as desired.