make --config a global option on rita command

[vivek-26]

This PR does the following:

• makes --config a global option on the rita command.
• removes --config flag from subcommands (import, html-report etc.)
• uses GlobalString to fetch config value from global state. For reference, see this issue.

Closes #595

[ethack]

This is great, thank you!

Do you think you could also migrate the test-config command to use the global option?

$ ./rita test-config --help                            
NAME:
   rita test-config - Check the configuration file for validity

USAGE:
   rita test-config [command options] [arguments...]

OPTIONS:
   --config value, -c value  specify a config file to be used

---

With these changes the config flag isn't recognized after the command. This invocation is what was needed prior to these changes and now produces an error. This could be a breaking change for some users.

./rita show-beacons --config etc/rita.yaml test
Incorrect Usage: flag provided but not defined: -config

Do you know if there's a urlfave/cli option to have the --config flag recognized wherever it is placed?

If there's not a simpler solution than putting the config flag back into all the subcommands and testing each case for both global and local config flags then I would rather we just announce the breaking change and possibly hold this until a major version bump. But if there's a clean fix that lets the flag be recognized anywhere then I'd be up for that to maintain backwards compatibility.

[vivek-26]

Do you think you could also migrate the test-config command to use the global option?

Sure, will do that.

Do you know if there's a urlfave/cli option to have the --config flag recognized wherever it is placed?

@ethack As per this issue, there are 2 ways to achieve this. We could do something like this or we can upgrade urfave/cli to v2. I wrote a very simple example using urfave/cli-v2 and global flag is recognised by all subcommands. However, upgrading urfave/cli to v2 might cause breaking changes. (example)

[vivek-26]

@ethack I have updated PR to make --config flag available to both root command and all subcommands.

Current CLI behaviour is as follows -

---

Command: $: ./rita --config /etc/rita.yaml test-config
Behaviour: test-config subcommand is aware of the --config flag value passed to the root command.

---

Command: $: ./rita test-config --config /etc/rita.yaml
Behaviour: test-config subcommand uses --config flag passed to it.

---

Commands:
alias rita1=rita --config /etc/rita/config.yaml
$: rita1 test-config --config /etc/new-config.yaml

Behaviour: test-config subcommand overwrites --config value set globally - /etc/rita/config.yaml and uses /etc/new-config.yaml.

---

Closes #595

[vivek-26]

@ethack PTAL

[ethack]

LGTM. Thanks for the contribution!