Support postgrator v5, add cosmiconfig, etc

[perrin4869]

Past few weeks I've been working on an update to this lib since I wanted to use it in my current project.
It brings new features, fixes, supports latest versions, etc. I updated the CHANGELOG.md:

* (**Breaking**) Node.js 12 or later required
* (**Breaking**) Removed option -t, --detect-version-conflicts. Conflicts are now always detected.
* (**Breaking**) Target postgrator 5.x.x
* (**Breaking**) Migrate to using cosmiconfig for configuration file
* (**Breaking**) `secure` option is now `ssl` and it works differently for each supported driver
* Add support for node 16.x.x
* Support merging cli and config file options

There are probably more changes/improvements that I forgot to mention.
My intention is to hopefully release this as v5 or make it a starting point for v5 of this lib.
For your consideration!

[perrin4869]

Added a new command called drop-schema and a new method to call migrations via postgrator migrate --to=max, etc. This is all backwards compatible because when no command is specified, it defaults to postgrator migrate, and if an integer is passed as the command (e.g. postgrator 5) it will default to the migration command. This can be a starting point to adding new features such as #8 via new commands (e.x., postgrator new --description, etc)