Feature/36 42 config improvements

[andscoop]

• closes Add --global / -g flags to astro config set/get #36 adding -g flag
• closes Rename config.json to astro.json to avoid old CLI conflicts #42 changing json config to yaml
• refactors airflow checkForProject to use PreRun Hooks
• moves default config setting to cfg struct

[schnie]

PreRun stuff all looks great, just have some questions about the config structure / patterns.

[schnie]

@andscoop this method signature is starting to feel a little off. Same for the init block of callers above it.

What if we added a NewCfg method to meta.go that acts as a "constructor" and returns a new cfg object, with default values. Seems like a lot of this the invocations of this method use the same defaults. Maybe we could cut out some boilerplate, add some default values, and let callers of NewCfg directly override specific values by name. Something like

PostgresUser := NewCfg(cfg{Name: "postgres.user", Default: "postgres"})

Really comes down to moving the initCfg function to live with the cfg type definition and using the NewXXX pattern. Rather than having a seemingly arbitrary list of parameters to pass every time, we can set default values, that are overridden specifically by name, rather than just appearing in a list.

[schnie]

Maybe we should rename meta.go to types.go as well. Then we could have multiple Cfg types that maybe share some group of defaults. For example, GettableCfg, SetableCfg. Not sure if multiple types is really needed yet.

Also, do we need a setD param here? Could we get away with just checking if the Cfg has a Default value set when created and call setDefault if it's not nil or an empty string?

[andscoop]

@schnie pushed change that

• removes gettable and settable props as we discussed offline about them being pointless for now. all configs should be gettable and settable - otherwise we will hardcode them
• renames meta.go to types.go