Add support for model tag selection in cli --model

[abhimskywalker]

The default model tag is 0.1.0-small. If we want to use the base model (or other tags in future), we should be able to use 0.1.0-base as a model tag liek this:

$ nougat path/to/file.pdf -o output_directory -m 0.1.0-base

[abhimskywalker]

I tried parsing a few PDFs with small version but it gave MISSING_PAGE on some of the pages, while the base model was able to convert the pages perfectly.

So have raised #60 as a PR towards this support for model selection at cli level to help both myself and others. :)

I could have perhaps just opened the PR and not an issue independently, but wanted to ensure this issue captures any other user's concerns around this if they have similar concerns. Apologies in advance if this increased any workload.
Have linked the issue to PR so this should close along with the PR whenever it's merged.