Improve security by not reading files

[bgaillard]

As mentioned in #3 versions 1.0.0 and 1.0.1 of the plugin prevented plugins to be triggered with auto-complete actions (i.e. while editing text) but not when a file was opened.

This PR changes how the plugin works to secure files even more by simply not reading them.

⚠️ The PR is currently in "work-in-progress", see the todo list bellow to know what work still needs to be be done before merging it.

TODO

• Remove the secured_files option
• Describe in the README.md file how to use the patterns and add links to the Neovim documentation for those who need details and more "complex" configurations
• Create a content for the readonly buffer with the exact Neovim command to call to open the file securely
• Explain how the plugin works in the README.md file
• Update the CHANGELOG.md file