Firstly, it's useful to install Plugin Manager. It's in the form of a .DLL that goes into the standard
plugins directory, and an .EXE that goes into the
MarkdownViewerPlusPlus provides a preview pane that renders your Markdown file (though not so prettily. Markdown_npp provides syntax highlighting.
You can select by columns! Either with
Alt + Ctrl + arrow keys or
Alt + leftmouse + drag.