Improve conversion script interface

`gargantext-cli ini` now takes two optional arguments:
- `--ini-path somefile.ini`: will read the configuration from `somefile.ini`.
  Defaults to `gargantext.ini`
- `--toml-path somefile.toml`: will write the configuration converted to TOML
  into `somefile.toml` (warning: it will erase the former file if there is already one!)
  Defaults to `gargantext-settings.toml`

Since it writes to a file, the command no longer outputs the configuration
to standard output.