Don't build if no files changed (and same build command)

Right now Ionic CLI builds all the time. It would be nice if the CLI could skip a build if no files were changed and the build in `www` was done with the same params as the current command.

Start at the Ionic CLI build command and trace how it detects changed files and records the parameters used for the existing build in www. Done means the CLI skips the build only when no files changed and the current command uses the same parameters; otherwise it builds normally.

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