Add mono-runtime to support "rawrr" package

[bnmoore]

I would like to request adding mono-runtime to the server.

I was trying to publish a shiny app on shinyapps.io that uses the "rawrr" package (from Bioconductor). This package uses .NET to access mass spec raw data files and for Linux it requires Mono. The error message is below.

Error: package or namespace load failed for ‘rawrr’:
.onLoad failed in loadNamespace() for 'rawrr', details:
call: fun(libname, pkgname)
error: The cross platform, open source .NET framework (mono) is not available.
Consider to install 'apt-get install mono-runtime' on Linux
or download/install from https://www.mono-project.com/.