Added ability to show unit in fixed width tables

Example usage:

```
t = Table()
t['a'] = [1,2,33133]
t['b'] = [1293494., 11., 1./3.]
t['c'] = ['a','c','wqdqwd']
t['a'].unit = 'mJy'
t['c'].unit = 'ergs/cm^2/s/Hz/ster'
write(t, 'test.txt', format='fixed_width', header_unit_line=True)
```

gives:

```
|     a |              b |                   c |
|   mJy |                | ergs/cm^2/s/Hz/ster |
|     1 |      1293494.0 |                   a |
|     2 |           11.0 |                   c |
| 33133 | 0.333333333333 |              wqdqwd |
```

@taldcroft - the motivation for my question in https://github.com/astropy/astropy/issues/2866#issuecomment-52654024 is the following: how do I change the above script to easily change the delimiter? Can we add the default delimiter as an option in ``__init__``?

cc @eteq @taldcroft @hamogu