Unnecessary files in npm package

The `0.4.0` version of the package contains some unnecessary files which makes simpleheat as a dependency larger than it needs to be. For example, the `demo` directory (14kb) and `yarn.lock` file (27kb) are unnecessary in the released version.

You could easily exclude these by adding an `.npmignore` file into the root before packaging:
```
.npmignore
demo/
yarn.lock
```