[Feature] Add nested struct/*struct and embedded struct

[jakefolio]

This PR adds support for nested structs, struct pointers, and embedded struct. Since this PR: #2 looks to have gone silent, I went ahead and made the requested changes (documentation, tests, etc.)

[ianlopshire]

@jakefolio Thanks for taking the time to submit this! I'll try to carve out some time this week to give this a proper review.

At first glance I have some questions:

How are paths handled for nested structs? To make nested structs really useful I think they must inherit the path from the parent struct.

type DBConfig struct {
    User     string `ssm:"user"` 
    Password string `ssm:"password"` 
}

type Config struct {
    PrimaryDB   DBConfig `ssm:"db/primary"` 
    SecondaryDB DBConfig `ssm:"db/secondary"` 
}

Does this implementation support nesting structs arbitrarily deep?

[jakefolio]

Please correct me if I'm wrong, but I believe you're asking about SSM path inheritance? If so, I believe your example above has the expectation of DBConfig.Password having the SSM path of db/primary/password and db/secondary/password, correct?

This PR does not include that type of nesting, but that is an interesting feature that I believe is beyond the scope of the PR.

This PR allows for the parsing of nested struct and struct pointers like the example in the test:
https://github.com/ianlopshire/go-ssm-config/pull/8/files#diff-5004671fb5eff27da0c3f2ec0bd1cb5a250f7287004872e8f5f125073e86606dR34