support SUMMARY.md for table of contents

[pietroppeter]

trafficstars

• feature present in mdbook: https://rust-lang.github.io/mdBook/guide/creating.html#summarymd
• SUMMARY.md gives a way for the user to specify the table of content using a plain markdown file
• in this way there is no more need to specify a table of contents in a nim source file and we could directly provide a nimibook executable

I think we should go in this direction and we are actually pretty close.

[pietroppeter]

make sure we initialize a default summary document