Adding html table conversion support for markdown files

Hi,
    When editing a markdown file, I realized it would be really useful to have the convert to table plugin work for it. Actually I assumed it would work as markdown files support embedding html too.  As this did not work, I made a small change to tableconvert.c to support html tables in markdown files.  I compiled the latest of geany (Geany 1.24 git >= ef33175) and geany-plugins with the change and it worked very well.  I hope you will find the change useful.