Reorganize GFA operations into ops and cli modules

[sampsyo]

Closes #191. This goes in a somewhat different direction: it puts the core logic of the nontrivial command into ops and leaves all the CLI stuff (argh structs, odgi-style option string parsing, odgi-style TSV printing) in cmds. The latter goes in a new cli module.

So now the module hierarchy goes like this:

flatgfa
  ::ops::*  // various transformations and analyses
  ::cli::*  // an odgi-like command-line interface
  ::*  // the actual data structure, etc.

Someday it may make sense to separate the CLI into a separate crate?