Make highlight optional

[hadley]

Since it isn't needed by most users (who use RStudio), and development status is uncertain.

[nturaga]

Yes, "highlight" is also not available for the R-devel version now, which makes lookup unavailable.

R CMD INSTALL lookup
* installing to library '/Users/ni41435_ca/R-stuff/bin/R-devel/library'
ERROR: dependency 'highlite' is not available for package 'lookup'
* removing '/Users/ni41435_ca/R-stuff/bin/R-devel/library/lookup'

> BiocManager::install("highlite")
Bioconductor version 3.9 (BiocManager 1.30.3), R Under development (unstable)
  (2018-10-30 r75519)
Installing package(s) 'highlite'
Warning message:
package 'highlite' is not available (for R Under development)

[jimhester]

@nturaga please install lookup with devtools::install_github("jimhester/lookup") as suggested in the readme, it will automatically install the correct highlite package.