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.

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

Test Results

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

========================================
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
========================================

Closes #138