feature: determine parameter name from field name

[briskt]

I have been using kelseyhightower/envconfig for some time. One feature that is very helpful in that package is its ability to automatically assign a field from an environment variable of the same name. While that is a nice feature, it is at odds with the design of go-ssm-config in that this package specifically ignores any field that doesn't have an ssm tag. Would it be undesirable to change that specification by adding another tag like ssm-auto that activates automatic naming? Or simply use automatic naming if no tag is found. The latter would be a bigger breaking change, but a simpler API.