Add `altima configure` command (Issue #11)

This closes #11 (closed).

Disclaimer: I am very new to Go, so if there is anything that can be refactored into something more idiomatic, please let me know!

I have marked this as a draft because there are a few open questions:

• The current implementation assumes that a file "altima.toml" already exists and lives in the same folder as the altima executable. What is the best place and name for the "altima.toml", and is it reasonable to assume that this file exists or should it be created if necessary?
• Reading and manipulating the TOML would be much simpler if it had a known structure (including types). I assumed that this is not the case, because modules should be able to create whatever keys and types of values they require. Is that correct? Because of this, though, the types need to be deduced. Currently only the following types are supported (all other types will be treated as strings):
  • string
  • bool
  • int (may get converted to float if part of an array)
  • float
  • array (including mixed and/or nested)