Add a flag to download the model

[thepushkarp]

Currently, if the model is not downloaded, the program downloads it during the first run, in the middle of the "Retrieving the commits ..." status mesage.

This can be improved by adding a flag through which the user can download/redownload the model when they need it.

Additionally, the program should prompt the user when it is run without downloading the weights with a choice to download it now or to abort the program.

[thepushkarp]

the sentence_bert currently does not have explicit control on when to download the models. When loading the models, it downloads if it does not find it already.

Therefore, in #21, I have loaded the model before the "Retrieving the commits ..." status message so that the download progress bar on first use can be seen clearly.