read_html and read_json should provide a cache
Hello,

`pandas.io.html.read_html` should provide a cache such as what requests_cache provides
https://requests-cache.readthedocs.org/en/latest/

persistence into sqlite, mongodb, redis... is very convenient.

Maybe some other pandas functions could also use this cache mechanism.
`pandas.io.json.read_json` for example

This is what I do

```
import requests
import requests_cache
expire_after = 15*60 # expire_after=delay_second or None
requests_cache.install_cache('req_cache', backend='sqlite', expire_after=expire_after)
import json
import pandas as pd
from StringIO import StringIO

req = requests.get(url)
io = StringIO(req.content)
df = pd.read_html(io)
```

Kind regards
