Plain text / Terminal formatter

I thought I would open an issue to gauge interest in a terminal / plain text formatter. This possible formatter would print or read data from space padded plain text.

```python
import tablib

data = tablib.Dataset()
data.headers = ('first_name', 'last_name')
data.append(('John', 'Adams'))
data.append(('George', 'Washington'))
print data.export('txt')
# first_name last_name
# John Adams
# George Washington
```

If you think this is useful and fits with the philosophy of the project, I could put together a PR.

There are definitely some differences between this and some of the other formatters including:

- how wide should it print by default?
- if the data is wider than the width, there will be data truncation
- this can't really losslessly handle (outputting and then reading a dataset will result in differences) data with spaces at the beginning or end