feat: add an optional cache that speeds up consecutive runs

[sod]

used like:

ngx-translate-extract --cache-file node_modules/.i18n-cache/my-cache-file --input ./src --output ./src/i18n/{da,en}.json
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In our project with 1100 scanned files, cached runs go down from 2 seconds to 0.3 seconds. And the terminal is less spamy as it only reports changed files.

The cache is implemented as careful as possible:

• opt-in, so disabled by default (you need to provide --cache-file option)
• the only thing that is cached is: "filename + contents" => "parser result"
• removes entries from cache that were unused, to not let stale data linger
• invalidate cache if package.json changes (aka new ngx-translate-extract release)
• the globbing/search for files isn't cached, the extractor still looks at the entire project from scratch

But even if you don't use the cache, uncached is faster as well, as the .union calls that I removed shaved off half a second as well for us.

A cached run looks like this:

[michaelbromley]

Thank you for this thorough implementation of a great new feature! I'll do a minor release which includes this.