Use chicken in vignettes/README

[stephenturner]

This PR changes out previous examples, README, vignettes, etc. to use the chicken genome, transcript db, and org db, all of which already exist in Bioconductor and can be installed with BiocManager.

Fixes #11, sets up for a manuscript describing tfboot.

@tabbzi @AlexanderTitus tagging you for code review for awareness only. Once I have all CI/CD checks passing ✅ I'm going to merge this in.

Local devtools::check() is clean

── R CMD check results ──────────────────────────────────────────────────────────────────────────────────────────────────── tfboot 0.2.0 ────
Duration: 1m 55.3s

0 errors ✔ | 0 warnings ✔ | 0 notes ✔

R CMD check succeeded