fix watch-compile path

Give file tree like this:
```
➤➤ tree
.
|-- dir
|   `-- index.jade
`-- index.html
```
or:
```
➤➤ tree
.
|-- dir
|   `-- index.html
`-- index.jade
```
And command `jade --out ./ -wP dir/index.jade` or `jade --out dir/ -wP index.jade`.
The path `monocle` returns is not clear enough and that leads to an error in #974 
I'm using `path.relative` to rewrite the path.