feat(#138): added configurable fts and exts

[AlejandroSuero]

With this PR we achieve a more personal configuration while keeping the defaults if the user doesn't want to change the behaviour in relation to filetypes and extensions.

• Added configurable filetypes and extensions
  • Added extra_filetypes and extra_extensions for better user experience (keeping the defaults while extending them)
• Updated documentation
• Updated tests

Test Results

Starting...
Scheduling: tests//glow/glow_spec.lua

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Testing:        /path/to/nvim_plugins/glow.nvim/tests/glow/glow_spec.lua
Success ||      setup setup with default configs
Success ||      setup setup with custom configs

Success:        2
Failed :        0
Errors :        0
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Closes #138