Allow calling #write twice

[denisdefreyne]

Fixes #1037. Allows calling #write multiple times.

Steps:

• Allow snapshot actions to have multiple names
• Allow snapshot actions to have multiple paths
• Allow item reps to have multiple paths per snapshot
• Merge consecutive snapshot actions into one
• Assign unique names to snapshot actions with no names

[denisdefreyne]

Forward path references continue to be a pain… e.g.

compile '/foo.*' do
  snapshot :raw
  filter :relativize_paths, type: :html unless rep.path.nil?
  write item.identifier.without_ext + '.html'
  snapshot :last
end

(:raw and :last snapshots added for clarity.)

The issue is here is that rep.path is hard to predict. If it were to return a string, then the :last snapshot will have a path, while the :raw snapshot does not. This is in line with expectations.

Now consider this example, where write and filter are inverted:

compile '/foo.*' do
  snapshot :raw
  write item.identifier.without_ext + '.html'
  filter :relativize_paths, type: :html unless rep.path.nil?
  snapshot :last
end

If rep.path were to return a string, then the :last snapshot will not have a path. Inversely, if rep.path returns nil, then the :last snapshot will have a path. This contradicts itself!

[denisdefreyne]

I’ve decided to not deal with the non-convergence issue yet. I believe this could be detected (i.e. if the paths have not yet converged after two attempts at calculating them, raise an error).