Add option to ignore leading/trailing whitespace inside <a> tag

- Version by `html2text --version`

2020.1.16

- Test script

```
echo 'Visit: http://example.com ' | html2text
```

This produces:

```
Visit: [ http://example.com ](http://example.com)
```

but it would be nicer if it produced:

```
Visit:
```

I recognise that theoretically the whitespace can be meaningful to the rendering of the HTML, but it just as often isn't, and having an option to ignore it would be nice. (In fact, I'd suggest that option should be the default.)

- Python version `python --version`

Python 3.7.5