When assigning values to flags (also when interogatting via
`context.(Global)IsSet`.
For boolean flags, consider empty as `false`.
Using `syscall.Getenv` rather than `os.LookupEnv` in order to support
older Golang versions.
Previously, defaults specified as nested keys in a yaml file would not
be recognized, i.e. `top: \n bottom: key` would not be accessible using
the name `top.bottom`, but `top.bottom: key` would. These changes
support using nested keys by traversing the configuration tree if the
key name uses '.' as a delimiter.
* fix typo (yamlSourceLoader -> yamlSourceContext)
* fix function return type (NewYamlSourceFromFlagFunc)
Also fixed build tags to properly build on >=go1.2
Signed-off-by: Jesse Szwedko <jesse.szwedko@gmail.com>
