Warn if build/dist directories already exist

[dannyadair]

Especially with custom locations #20 it is possible that a user can affect unrelated files when:

• creating files during build
• deleting "all files" during build teardown
• creating files in dist
  (Repeated runs are expected to overwrite with new PDF versions in dist.)

Warn the user if build/dist already exist and ask for confirmation.
Provide an option to always say "yes" to these.

[dannyadair]

Two problems - more than one set of directories and re-runs expecting them to exist:

• there is a top-level build+dist but the user can actually configure different build+dist for each document, so this can get complicated and also make for an annoying program flow
• build directories may be kept (with --keep-build or --debug) and of course a re-run will see an existing build directory
• similarly, the user will be editing configs and re-running to look at new versions in the dist directory

So for the sake of re-runs, I will not do this check.
Instead #14 pdfbaker validate will check this and warn.