Gemroff v8

- Gemroff.Main: Handle rendering from Standard_Input properly
- Gemroff.Main: if no input arguments provided, use standard input
- README: update install instructions
- README: separate alire and normal build instructions
- README: update and reorganize build instructions to make clear the preferred
  options, and which to choose
- gemroff(1): fix incorrect example (missing output format)
- gemroff(5): fix bad HTML escape in rendered output
- docs: add additional known issue when multiple input files are specified
- docs: remove superfluous stdout output file in known issues example
- Makefile: don't install-library by default, add alire-install target
- alire.toml: add test action to run tests
- alire.toml: fix version number to follow semver and allow compiling
- alire.toml: add executable.gpr to project-files array so Alire actually
  builds binary
- .builds: add fixed debian-alire buildscript to test alire building