gistfile1.js

// assumptions:
// link = the markdown to inject
// textarea = jQuery() of the node

textarea.val( function( _, value ) {
    var range,
        begin = value.length;
    if ( this.setSelectionRange ) {
    	begin = this.selectionStart;
    } else if ( document.selection && document.selection.createRange ) {
    	range = document.selection.createRange();

    	// the moveStart returns the number of characters it moved as a negative number
    	begin = 0 - range.duplicate().moveStart( "character", -100000 );
    }
    return value.substr( 0, begin ) + link + value.substr( begin );
});