Add save() and accept state in reset()

Closes #12.

moo.js

@@ -239,14 +239,21 @@
     return this.re.lastIndex
   }
 
-  Lexer.prototype.reset = function(data) {
+  Lexer.prototype.reset = function(data, state) {
     this.buffer = data || ''
     this.re.lastIndex = 0
-    this.line = 1
-    this.col = 1
+    this.line = state ? state.line : 1
+    this.col = state ? state.col : 1
     return this
   }
 
+  Lexer.prototype.save = function() {
+    return {
+      line: this.line,
+      col: this.col,
+    }
+  }
+
   Lexer.prototype.feed = function(data) {
     this.buffer += data
     return this

test/test.js

@@ -188,6 +188,27 @@ describe('line numbers', () => {
 })
 
 
+describe('state', () => {
+
+  const testLexer = compile({
+    word: /[a-z]+/,
+    NL: { match: '\n', lineBreaks: true },
+  })
+
+  test('can be saved', () => {
+    testLexer.reset('one\ntwo')
+    testLexer.lexAll()
+    expect(testLexer.save()).toEqual({line: 2, col: 4})
+  })
+
+  test('can be restored', () => {
+    testLexer.reset('\nthree', {line: 2, col: 4})
+    expect(testLexer).toMatchObject({line: 2, col: 4, buffer: '\nthree'})
+  })
+
+})
+
+
 describe('python tokenizer', () => {
 
   test("1 + 2", () => {