Allows port to be specified as an environment variable

[shikokuchuo]

Fixes #956 (tagged 'help wanted').

The error returned there is an Rcpp error, as the httpuv function that's ultimately called is expecting an integer input and can't handle the implicit character -> integer conversion. Environment variables are by default retrieved as strings.

I've added the as.integer() conversion as a catch-all, rather than add a conditional branch as:

i) I find it slightly more readable
ii) slightly more defensive, in case the methods to find a random port above it are updated
iii) getRandomPort() returns a double so there would have been an implicit conversion at the Rcpp level anyway

Includes a regression test.

PR task list:

• Update NEWS
• Add tests
• Update documentation with devtools::document()

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