Add support for configuration subsets

[skizot722]

Sometimes there may be a need for passing around subsets of a configuration to your methods. Accessing this section of a config can become very verbose and also a hassle, especially with deeply nested configs. This code change introduces the concept of "configuration subsets", hopefully making it easier to deal with. Please let me know what you think. I am successfully using this with my application, and I'm hoping it may be of benefit to others.

Configuration Subsets

An alternative to the GetStringMap / Materialized paths is a configuration subset. This allows you to create a new *confer.Config object which is a subset of the full configuration, specified by a key prefix. This makes passing around a subset of the configuration easy, allowing you to access the values using the getter methods listed above as you would normally. The setter methods are also supported on a configuration subset, as well as nested configuration subsets.

Example Config

---
my-app:
    logging:
        enabled: true
        level: debug
    server:
        workers:
            - 127.0.0.1
            - 127.0.0.2
            - 127.0.0.3
    database:
        bird:
            host:     feathers
            user:     postgres
            password: superl33+
        wind:
            host:     sea-spray
            user:     postgres
            password: easyPZ

Config Subset

windDbConfig := config.NewConfigSubset("my-app.database.wind")
host := windDbConfig.GetString("host")

Nested Config Subset

dbConfig := config.NewConfigSubset("my-app.database")
windDbConfig := dbConfig.NewConfigSubset("wind")
host := windDbConfig.GetString("host")

[skizot722]

Looks like there are issues with godep using tip right now:

tools/godep#452
tools/godep#453

Until that issue is solved, build using tip is not going to succeed on Travis CI.

[jacobstr]

Interesting. You're choosing to copy the data here rather than maintaining a reference. Would it surprise you to find that your subset was unchanged after modifying a value in the parent config?

[skizot722]

No surprise to me :-).This was intentional, as I only want to modify the subset of config passed. That's why I chose to prefix the function name with "new". I can change it to a reference, but the intent was to make a copy of the subset.

[jacobstr]

Cool. I'm curious what the use case here is? How are you using subsets?

[skizot722]

Subsets are used like in the example listed above. However in practice, the subset is nested much deeper.

[skizot722]

Hi @jacobstr is this something you'd be interested in?