feature: add support for eggnog-mapper

[franciscozorrilla]

🥚 eggNOG-mapper for better functional annotation 🍸

🐍 Snakefile tasks

• create eggnog-mapper rule for processing ORF-annotated protein fasta files of genomes
• create helper/supplementary rule to deal with large number of genomes (e.g. per sample submission of jobs)

🔨 Additional modifications for full integration/support

• metaGEM.sh: add options + support for new tasks
• config.yaml: add params used in new tasks
• config readme file: eggnog-mapper could go in main metaGEM env
• conda recipe: update
• main readme file: text and eventually figure