Add runtime check for Ghost version

[vickychijwani]

Quill will not work against all Ghost versions because the Ghost API is not stable and public yet. So we must add a runtime check that tests what version the user is running and inform them in case we don't support it / haven't tested against it. It might even make sense to refuse to proceed if the version is unsupported, to avoid messing with the user's data.

[vickychijwani]

Useful API endpoints:

In 0.6.3:

GET /ghost/api/v0.1/configuration/ HTTP/1.1

HTTP/1.1 200 OK
{
    "configuration": [{
        "key": "version", 
        "value": "0.6.3"
    }]
}

In 0.7.9:

GET /ghost/api/v0.1/configuration/about/ HTTP/1.1

HTTP/1.1 200 OK
{
    "configuration": [{
        "database": "sqlite3", 
        "environment": "development", 
        "mail": "", 
        "version": "0.7.9"
    }]
}