Config refactor

[gravestench]

depends on #33!

• config global state removed from the config module
• config global state now resides in main, TODO to remove it
• config now creates a default if none are found
• added command line option for specifying the config file
• added command line option to specify the log level
• config directory now looks in user dir, in a platform agnostic way. ex: on linux, will typically look in ~/.config/my5g/RANTester for file config.yml
• updated go.mod to use go v1.16
• embedded default config file data
• errors are now bubbled up appropriately