Support configuration file for export command

[WordlessEcho]

Resolves #54

If both command parameters and configuration file is specified. It will respect the options of command parameters.
Improvement: Maybe we should make default value of command parameters as constant?

Tested on my environment by following configurations:
~/config.json

{
    "outputDir": "~/export-test",
    "channels": ["t"]
}

go build
./inter-server-sync export --config=~/config.json --channels=testchannel

Result:

$ cat ~/export-test/exportedChannels.txt
testchannel

[WordlessEcho]

About 5be4e6d : ServerConfig is belongs to cmd/root.go. I think we should not change it in cmd/export.go?

[avshiliaev]

I like your proposal, we could definitely improve it a bit and include into our project if you'd like top work on it.

[avshiliaev]

That's a nice approach. I'd like it to be a bit more explicit on the user interface level, that we for example can either provide a config or specify it as a set of args. This way we could probably not mix them together with the options. What do you think?

[WordlessEcho]

@avshiliaev Sorry. I didn't catch your point... Can you give me some examples?
Or I guess that you hope the --config could be global flag as same as --serverConfig?

[WordlessEcho]

I found some bug in recent commits. Waiting for fix.

[WordlessEcho]

Bug fixed.
About the --config option: I wrote some codes to move it into cmd/root.go as global flag but not pushed yet.