refactor initialization and allow to specify custom binary by config

[doits]

• It allows to set a custom MJML binary simply by doing Mjml.mjml_binary = ... in an Rails initializer, no warnings are printed anymore
• The custom binary will be checked and an error is raised if it cannot be run correctly
• The search and check for a valid binary is run after Rails' initilization process now to allow the configuration of the custom binary

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Edit: If this is merged it should be released as a new major version, even though I tried to make sure not to break existing apps that use Mjml::BIN and Mjml.discover_mjml_binary.

[doits]

I added some methods to make upgrading existing apps easier which use Mjml::BIN to set the mjml binary. So old apps should not break anymore.

[sighmon]

@doits Looks good, and works locally.

[chasegiunta]

Thank you for this @doits!!