Add a `--no-debug-vars` command line switch to disable the `/debug/var` endpoint

[dee-see]

Description

Fixes #66

Checklist

• [x] Code compiles correctly
• [ ] Created tests (if appropriate)
• [ ] All tests passing
• [x] Extended the README / documentation (if necessary)

[dee-see]

The tests are actually failing for me locally but it's not related (I think?)

$ make test
Running tests...
?       github.com/cactus/go-camo/v2/cmd/go-camo        [no test files]
?       github.com/cactus/go-camo/v2/cmd/url-tool       [no test files]
--- FAIL: TestUpstreamProxyParsing (0.00s)
    upstream_proxy_test.go:53: assertion failed: 
        --- addresses
        +++ →
          []string{
                "127.0.0.1",
        +       "::1",
          }

--- FAIL: TestUpstreamProxyParsing (0.00s)
    upstream_proxy_test.go:53: assertion failed: 
        --- addresses
        +++ →
          []string{
                "127.0.0.1",
        +       "::1",
          }

FAIL
FAIL    github.com/cactus/go-camo/v2/pkg/camo   6.197s
ok      github.com/cactus/go-camo/v2/pkg/camo/encoding  0.006s
ok      github.com/cactus/go-camo/v2/pkg/htrie  0.006s
ok      github.com/cactus/go-camo/v2/pkg/router 4.007s
FAIL
make: *** [Makefile:82: test] Error 1

[dropwhile]

@dee-see yeah, that test can fail in some environments and is unrelated. PR looks good to me. Thanks!