add `options.parsePlus`: Use `buf +=` instead of `buf.push()`

Add a new `parse` function, using `buf +` instead of `buf.push()` for string concat.

It will improve performance.

My benchmark result:

```bash
$ node benchmark2.js

renderNoCompileDebugNoWith source:

var buf = [];
 buf.push('');1; if (locals.user) { ; buf.push('\n  <h2>hello, ', escape((2,  locals.user.name )), ' ', escape((2,  locals.user.not_exists )), '</h2>\n  <p>', (3,  locals.user.title ), '</p>\n');4; } ; buf.push('');
return buf.join('');
--------------------------------------------------------------
render_ParsePlus_NoCompileDebugNoWith source:

var buf = "";
 buf += '';1; if (locals.user) { ; buf += '\n  <h2>hello, ' + escape((2,  locals.user.name )) + ' ' + escape((2,  locals.user.not_exists )) + '</h2>\n  <p>' + (3,  locals.user.title ) + '</p>\n';4; } ; buf += '';
return buf;
--------------------------------------------------------------
render: 
  <h2>hello, fengmk2 undefined</h2>
  <p></p>

renderNoWith: 
  <h2>hello, fengmk2 undefined</h2>
  <p></p>

renderNoCompileDebug: 
  <h2>hello, fengmk2 undefined</h2>
  <p></p>

renderNoCompileDebugNoWith: 
  <h2>hello, fengmk2 undefined</h2>
  <p></p>

render_ParsePlus_NoWith: 
  <h2>hello, fengmk2 undefined</h2>
  <p>undefined</p>

render_ParsePlus: 
  <h2>hello, fengmk2 undefined</h2>
  <p>undefined</p>

render_ParsePlus_NoCompileDebugNoWith: 
  <h2>hello, fengmk2 undefined</h2>
  <p>undefined</p>

render() x 150,062 ops/sec ±0.93% (96 runs sampled)
renderNoCompileDebug() x 173,700 ops/sec ±1.32% (90 runs sampled)
renderNoWith() x 315,558 ops/sec ±1.19% (91 runs sampled)
renderNoCompileDebugNoWith() x 354,070 ops/sec ±1.51% (90 runs sampled)
render_ParsePlus() x 183,668 ops/sec ±0.76% (96 runs sampled)
render_ParsePlus_NoWith() x 547,998 ops/sec ±0.94% (93 runs sampled)
render_ParsePlus_NoCompileDebugNoWith() x 608,255 ops/sec ±0.92% (95 runs sampled)
Fastest is render_ParsePlus_NoCompileDebugNoWith()
```