Rename package to `rx`

[dmi3kno]

Awesome short name and a hex with blue and red pill. Tagline: "Rx is a painkiller for regex"

available::available("rx")
#> -- rx --------------------------------------------------------------------------
#> Name valid: ✔
#> Available on CRAN: ✔ 
#> Available on Bioconductor: ✔
#> Available on GitHub:  ✔ 
#> Abbreviations: http://www.abbreviations.com/rx
#> Wikipedia: https://en.wikipedia.org/wiki/rx
#> Wiktionary: https://en.wiktionary.org/wiki/rx
#> Urban Dictionary:
#>   (noun)from the symbol meaning "[prescription]" this [seeks] to label  someone very annoying that can only be taken in [small doses] at set  periods of time.
#>   http://rx.urbanup.com/915152
#> Sentiment:???

Btw, vx is also available name, if you decide to go for that one. But I really like the "painkiller" message