Fix examples, read_vcf, and dependency management

Misc. fixes in prep for public release / publication. Tagging a few reviewers for awareness but I'll probably merge this soon after CI/CD checks out.

Use simulated chicken in all @examples where previously used "Rosie" requiring a nonstandard package to run vignette examples fixes #13 via a9f2d26fbdb6c6ff395027639818976ac1e22778
Rewrite read_vcf() function to no longer use motifbreakR::snps.from.file() and use VariantAnnotation::readVcf() instead, followed by some post-processing. Fixes #10 via e934a51b3c4de4e91190d2704e2a7e4fc29966ee. Thanks 🎩 @tabbzi
Move motifbreakR to Suggests avoiding the hard dependency, since motifbreakR isn't technically a requirement to install and use tfboot. Fixes #14 via 73c50ddcd7c5cc0523d33733fd8061a76c7028ae.
Added a changelog / NEWS.md via 8ee016df67a16e902218ae420dc145dda8329bcf.

Local check OK:

── R CMD check results ───────────────────────────────────────────────────────────────────────────────────────────────────────── tfboot 0.2.1 ────
Duration: 1m 56.4s

0 errors ✔ | 0 warnings ✔ | 0 notes ✔

R CMD check succeeded

Noting that the motifbreakR dependency was "heavy" based on this dependency graph (bottom row). In all likelihood, a tfboot user will likely have motifbreakR installed, but strictly speaking, it's not necessary to run a tfboot analysis on precalculated motifbreakR results, without needing the motifbreakR dependencies.

colossal-compsci / tfboot

Fix examples, read_vcf, and dependency management #15