Added installation latest version of chrome and refactored fetcher

[initprism]

Fixed fetcher as discussed in
#374 (comment) and #374 (comment)

Now Revision::Latest gets the revision and installs the latest version of Chrome on that platform.

e.g

// let revision = headless_chrome::Revision::Specific("634997".into());
let revision = headless_chrome::Revision::Latest;
let fetcher_options = headless_chrome::FetcherOptions::default()
    .with_revision(revision);

let launch_options = headless_chrome::LaunchOptionsBuilder::default()
    .fetcher_options(fetcher_options)
    .build()?;

let browser = Browser::new(launch_options)?;

[mdrokz]

Looks good thanks for your contribution @initprism !