Adds PHENOSCAPE_API environment variable
Adds PHENOSCAPE_API environment variable

Adds PHENOSCAPE_API environment variable that will override the
base API URL when set. The default API URL is
https://kb.phenoscape.org/api.

Running tests with the dev instance can be done like so:
```
PHENOSCAPE_API=https://stars-app.renci.org/phenoscape-dev-api Rscript -e 'devtools::test()'
```

xu-hong/rphenoscape