Increased the compression level for MOBI ebooks

The new level is `c1`, as suggested by @magabriel in [this comment](https://github.com/javiereguiluz/easybook/commit/e4397cb2ef41be3fcb5f201c085e52ae2cf38b0d#commitcomment-3572369):

c0 compression produces a very large file and c1 is the more balanced,
while c2 is incredibly slow.