Add --cover option for creating cover with markdown file

## Create cover written by markdown
Because wkhtmltopdf has `--cover` option, you can create pdf with cover as follows.
```
gimli -f foo.md -w '--cover bar.html'
```
This pull request adds `--cover` option to gimli so that you can create cover written by markdown.

## Usage
```
gimli -f foo.md --cover bar.md
```
Then pdf will have a cover.
All texts in the cover will be center-aligned.