bazelisk.go: add simple .bazeliskrc file support
bazelisk.go: add simple .bazeliskrc file support

Replace `os.Getenv` with `getEnvOrConfig`, which falls back to parsing
key=value pairs from a `.bazeliskrc` file in the workspace
root. Environment variables take precedence.

Aims to solve #119 as simply as possible.

Closes #127.

bazelbuild/bazelisk,bazelbuild/bazelisk,bazelbuild/bazelisk,bazelbuild/bazelisk