Add margin-bottom to <table>

Adding a table to a post wouldn't have any margin below it, so the next
paragraph would be right up against the bottom.

tildes/scss/_base.scss

@@ -196,6 +196,7 @@ table {
   border-collapse: collapse;
   border-spacing: 0;
   width: auto;
+  margin-bottom: 1rem;
 }
 
 td, th {