Add a `blank_line()` method to `Builder`

Many times users may want to seperate lines of text using a blank line.  Currently, this can be accomplished by either adding '\n' to the previous `text()` call, which requires that the user has the ability to access this, and can also look a little messy, or by calling `text()` with an empty string, which definately works, but having an explicit method might be a nice sugar for a lot of users, and barely adds any weight to the codebase.

This is definately a small thing, and almost closer to a personal preferance than anything, but I definately think it would make the library a tiny bit nicer to use, and there's barely any tradeoff.  It's still up to you though if you'd rather keep your codebase small.

gemtext/src/lib.rs

@@ -18,6 +18,17 @@ impl Builder {
         self
     }
 
+    /// Append a single blank line to the document
+    ///
+    /// This is equivilent to calling [`text()`] with an empty string, or pushing a blank
+    /// [`Node`]
+    ///
+    /// [`text()`]: Self::text()
+    pub fn blank_line(mut self) -> Self {
+        self.nodes.push(Node::blank());
+        self
+    }
+
     pub fn link<T: Into<String>>(mut self, to: T, name: Option<String>) -> Builder {
         self.nodes.push(Node::Link {
             to: to.into(),