set a bunch of meta tags on package pages

Like so:

```html
<meta name="description" content="Fast, unopinionated, minimalist web framework">
<meta name="keywords" content="express,framework,sinatra,web,rest,restful,router,app,api">
<meta name="twitter:url" content="https://localhost:15443/package/express">
<meta name="twitter:title" content="npm: express">
<meta name="twitter:description" content="Fast, unopinionated, minimalist web framework">
```