Patch lektor-i18n-plugin to fix having to build a site multiple times

I've had a patch to lektor-i18n-plugin sitting around half-done for around three months. I'm currently working on finishing it up, and I wanted to make an issue here just to let everyone know I'm working on it.

Progress:

• patch the I18NPlugin class
  • turn all the necessary methods into class methods (__parse_source_structure, __trans_linewise, __trans_parwise)
  • patch the rest of the class to call these
  • move the on_before_build method body to a new translate_contents method, and have on_before_build call the new method
• write an example script that can be run once to translate all the contents, without having to build the site
• document the changes and script
• send the patch upstream

These changes won't actually break any CI/scripts/workflows that build the site multiple times, and it will enable a much faster and simpler workflow of ./translate_contents && lektor build

Reasoning: Our CI time for lektor sites is the three lektor builds, with a small overhead for the job to get started. This patch would cut down CI time to about a third of what it is now, saving anywhere from 2 to 10 minutes for a job to run. I also find myself having to run a completely clean build from time to time, so this saves local dev time quickly iterating changes.