Ninja mode.el refactor

An attempt to make `ninja-mode.el` slightly less bad.

The changes are mostly about making `ninja-mode.el` a good Emacs citizen, with no major changes. Most of it is style cleanups, except
```
0a0ee58 	Set up a proper syntax table for ninja-mode.
1bd6301 	Correctly recognize comments. 
```
which make `syntax-ppss` and everything relying on it behave sanely.

Should I squash the hell out of the commits?