add setpw and unsetpw for encrypting keys in fish.conf

I don't like storing keys in plaintext, so here are my changes to fix that. May be a bug or two, this is my first script for weechat. To test:

```
/blowkey setpw
```
Use as normal. Quit, restart, confirm keys work. Then:
```
/blowkey unsetpw
```
to return to storing as plaintext.