Add progress bar for "installing component" step

[johnthagen]

Some components, such as rust-docs and rust-src take a bit longer to install, even on SSD-equipped machines. These take even longer on Windows (#1540).

One thing that would help in these situations would be if the user could see the progress of the component install to:

1. Get a sense for how long the component will take to finish installing
2. Know that rustup isn't just hung up

Perhaps Cargo's new progress bar released in stable 1.30 could be reused in some way?

...
info: installing component 'rust-docs'
Installing [============>                                           ] 330/1400

[johnthagen]

At least one user reported thinking that rustup was stuck when it got to installing rust-docs: #1458 (comment)