tell newcomment.el # is only comment at line start

Otherwise we get things like this:
```
A typical commit message referencing issue #xxx that is long enough to
                                           #need filling,
```
This is not related to fontifying of comments which already applies only
to lines beginning with `#`. It's also not related to the syntax-table,
middle of line `#`s still have comment syntax, but apparently this has no
effect.

Fixes #67.