Why not enable `include_sup_sub` by default?

Hi,

I noticed that `include_sup_sub` is not enabled by default. This can lead to incorrect conversion of math expressions involving exponentiation.

For example, given the following code:
```python
import html2text

h = html2text.HTML2Text()
# h.include_sup_sub is False by default
h.handle("0 <= grid[i][j] <= 105")
```
The output will be:
```
`0 <= grid[i][j] <= 105`\n\n
```