support checking out individual files

[davidfirst]

Description

Currently, users can bit checkout components, but not individual files inside the component.
This is needed especially for binary files conflicts, where there is no way to edit the file and make decisions. By default, in case of conflicts, Bit keeps the existing binary file (unless --theirs was used). If a user chooses --manual to auto resolve-conflict and wants "theirs" binary file, it's not possible.

Describe the solution you'd like

I'd like to be able to run bit checkout latest comp-id --file button.ts

Describe alternatives you've considered

Coping the file manually.

Additional context

Asked by ranm8 .