Added option to stop inclusion of page content at a certain point.

[lpaulsen93]

This can be done by using the syntax tag {{includestop}} or by using the flag length.
E.g. if length=1000 is set, then only 1000 signs of a page will be included, counted is only cdata content.

Some more points to mention:

• if both {{includestop}} and flag length are used on the page then the earlier one wins
• this feature can be used to close a feature request in the blog plugin, see Please implement excerpts and a "read more..." macro plugin-blog#13.
• if a list is interrupted by the length flag or {{includestop}} then the current list content item and list is properly closed but no more list items are created
• if a table is interrupted by the length flag or {{includestop}} then the current table cell row and table is properly closed but no more table rows are created
• if the length flag or {{includestop}} are used then the read more link will be created if the flag readmore is set

[michitux]

In general I like the idea but I fear to get this to work properly there is more work needed. I have added a simple example to test case that does not work yet. Further, I think the current code doesn't close nested lists properly. Plugin support is also totally missing.

I have a different suggestion: what about disallowing the {{includestop}} syntax in nested environments, i.e., making this a syntax that only works in the top level or maybe in top level paragraphs. That should make the whole "close everything" code much simpler.