Add underscore to tokenizer identifier function (#75)

Allows for using underscore in variable names

Author: ape-sudo

.gitignore

@@ -1 +1,2 @@
 /target
+/.idea

src/tokenizer.rs

@@ -175,7 +175,7 @@ impl Tokenizer {
         loop {
             let chr = self.peek();
 
-            if chr.is_alphanumeric() {
+            if chr.is_alphanumeric() || chr == '_' {
                 literal.push(chr);
                 self.advance();
             } else {