[RFC] Features/configurable log name

So this is a quick and dirty stab at enabling a workflow I need: Two weechats running. One for a work server that likes to do lots of force joining and generally messing with my client, and one for all the other clients.

Before this, I was doing something like;

```bash
weechat
# Set that up for personal
mv ~/.weechat/weechat.{,personal.}log
weechat
# setup for work
```

but it's pretty awkward. With this patch I can just do

WEECHAT_LOG_NAME=work.log weechat and have it do the right thing. Thoughts?