Rework main parser logic

The way that we were doing it before was going to
make it very difficult to implement any sort of
error handling or extension to the language.

src/parser.js

@@ -15,30 +15,16 @@ module.exports = class Parser {
   }
 
   expr() {
-    this.tokenStream.eat(tokenTypes.OPAREN)
-
-    let node = new Node()
+    let node
 
-    if (this.tokenStream.peek().type === tokenTypes.LITERAL) {
-      node.type = 'functionCall'
-      node.functionName = this.tokenStream.eat(tokenTypes.LITERAL).value
-    }
-
-    node.args = []
-    node.subtree = []
+    this.tokenStream.eat(tokenTypes.OPAREN)
 
     while (
-      this.tokenStream.peek().type != tokenTypes.CPAREN &&
+      this.tokenStream.peek().type !== tokenTypes.CPAREN &&
       this.tokenStream.peek().type !== tokenTypes.EOF
     ) {
-      if (this.tokenStream.peek().type === tokenTypes.ATTRIBUTE) {
-        node.args.push(this.attribute())
-      } else if (this.tokenStream.peek().type === tokenTypes.OPAREN) {
-        node.subtree.push(this.expr())
-      } else if (this.tokenStream.peek().type === tokenTypes.LITERAL) {
-        node.subtree.push(this.identifier())
-      } else if (this.tokenStream.peek().type === tokenTypes.QUOTE) {
-        node.subtree.push(this.quotedString())
+      if (this.tokenStream.peek().type === tokenTypes.LITERAL) {
+        node = this.element()
       } else if (this.tokenStream.peek().type === tokenTypes.KEYWORD) {
         node = this.keyword()
       }
@@ -49,6 +35,29 @@ module.exports = class Parser {
     return node
   }
 
+  element() {
+    let elementNode = new Node({
+      type: 'functionCall',
+      args: [],
+      subtree: [],
+      functionName: this.tokenStream.eat(tokenTypes.LITERAL).value
+    })
+
+    while (![tokenTypes.CPAREN, tokenTypes.EOF].includes(this.tokenStream.peek().type)) {
+      if (this.tokenStream.peek().type === tokenTypes.ATTRIBUTE) {
+        elementNode.args.push(this.attribute())
+      } else if (this.tokenStream.peek().type === tokenTypes.QUOTE) {
+        elementNode.subtree.push(this.quotedString())
+      } else if (this.tokenStream.peek().type === tokenTypes.OPAREN) {
+        elementNode.subtree.push(this.expr())
+      } else if (this.tokenStream.peek().type === tokenTypes.LITERAL) {
+        elementNode.subtree.push(this.identifier())
+      }
+    }
+
+    return elementNode
+  }
+
   attribute() {
     let attributeNode = new Node()
     attributeNode.attributeName = this.tokenStream.eat(