add check-config flag

[glightfoot]

Fixes #31

@matthiasr

Signed-off-by: glightfoot [email protected]

[roidelapluie]

thanks! #145 would add a getool cmd line, which we could reuse for this?

[matthiasr]

:why_not_both: 😁

In general I agree that if there is a tool, it should support config checking. The --check-config flag is in line with the statsd exporter (that doesn't have a tool yet). We can also change it again later, "replace graphite_exporter --check-config with getool check config" is a kind of breaking change that I am not afraid to make.