[WIP] Add a -H/--hash-function option.

[ctb]

As per #1613 and #1511 (comment), this adds -H/--hash-function to countgraph creation. Among other things, this:

• adds a function khmer_args.create_counttable(...) analog of khmer_args.create_countgraph(...).

Some notes:

• we will need a load_counttable that properly picks up on khmer.Countgraph and khmer.Counttable.

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• Did it change the command-line interface? Only backwards-compatible
  additions are allowed without a major version increment. Changing file
  formats also requires a major version number increment.
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Do the changes respect streaming IO? (Are they
  tested for streaming IO?)

[ctb]

Update: it's pretty clear that this can't make it into 2.1 because we are going to have to change file formats to store what kind of object something is. Good to know :).