Added the possibility to change the default mappings

[bh-schmidt]

Allowed the users to override the default mappings.

Example:

local project_actions = require("telescope._extensions.project.actions")

require('telescope').setup {
  extensions = {
    project = {
      mappings = {
        i = {
          ['<c-r>'] = project_actions.rename_project
        }
      }
    }
  }
}