Add check to display outdated version warning

[alicse3]

Fixes #930

This change checks for outdated src-cli version and prints warning message for any command.

➜  src-cli git:(check-for-outdated-version) ✗ ./src
⚠️  You are using an outdated version 4.3.0. Please upgrade to 5.2.1 or later.
src is a tool that provides access to Sourcegraph instances.
For more information, see https://github.com/sourcegraph/src-cli

Usage:

	src [options] command [command options]

Examples:

Test plan

• Pull this PR changes and run cli commands directly(go run ./cmd/src) or by building go build ./cmd/src && ./src.
• To test different versions, change version.BuildTag value and re-run commands.