fix: clear title cache after rendering of a post

[renbaoshuo]

Problem

After #135, using the same heading in different posts caused the rendered posts' anchor id to be add unnecessarily suffixes.

Reproduction

_posts/post-1.md

---
title: Post 1
---

## Development

## Production

will be rendered to:

<h2 id="Development">Development</h2>
<h2 id="Production">Production</h2>

---

_posts/post-2.md

---
title: Post 2
---

## Development

## Production

will be rendered to:

<h2 id="Development-2">Development</h2>
<h2 id="Production-2">Production</h2>

actually the result should as same as _posts/post-1.md.

[renbaoshuo]

Should we release a new version after merge this PR?

[yoshinorin]

Should we release a new version after merge this PR?

Ready #185