Handle simple values

[mkatychev]

This PR adds "simple" primitive types as defined in IsSimpleValue to the "flat map" output:

// IsSimpleValue is intended to see if the underlying value allows a flat map to be retained
func IsSimpleValue(i interface{}) bool {
	switch i.(type) {
	case string,
		float32, float64,
		uint8, uint16, uint32, uint64,
		int8, int16, int32, int64,
		bool:
		return true
	}
	return false
}

Previously only map[string]string were considered to be "flat" associative arrays

So the presence of the integer 12 would mean that fetching the value for key1 necessitated declaring the read type as complex for all keys found in the map:

# file.yaml
key1: val
key2: {nested_key: val}

[file.vars]
key1.path = "file.yaml"
key1.type = "yaml{}"
key2.path = "file.yaml"
key2.type = "yaml{}"

This is no longer the case and flat values can also be mixed complex values without both needing the complex read type declaration:

[file.vars]
key1.path = "file.yaml"
# key1.type is deferred thus taking "yaml" from the file stem
key2.path = "file.yaml"
key2.type = "yaml{}"

[jproffitt]

Should we use a different name other than context? This makes me think of go's standard context.Context type

[mkatychev]

I thought about that a lot but I really think that the inherent meaning of the word here supersedes the namespace confusion it might cause. "Context" in terms of what you want the output of key value store to be is really important.

[jsgoyette]

Looks great!