Add `config` as language file

This patch will change several behaviour while generating the ebook:

- `build.go` now accepts two environment variables: TMP, WORKDIR

    * TMP:

Temporary directory path.
The directory stores intermediate files like '*.html' or metadata.txt.

    * WORKDIR:

Script base directory.

- `config` as language file

config file will be `source`d to change its ebook metadata.

- `genepub.sh` is integrated to `build.sh` and `build.go`

See `FixHeader` and `RemoveFooterLink` functions.