Add optional --fallback CLI option

[alex996]

Closes #164 and #166. Introduces an opt-in -f or --fallback command-line flag, which allows to serve up the start page as a fallback, when the requested route cannot be resolved. This is useful when developing a single-page app that handles routing client-side with HTML5 history API.

Currently, non-resolvable routes such as /about or /blog/some-post are falling through serve-static, which can't match them to a file, and thus returns a 404. When -f flag is on, reload will detect these routes, and respond with index.html (or custom start page when -s is provided). The front-end (e.g. react using react-router-dom) can then take over the rendering based on the URL path.

As a stretch goal we could also serve up blog.html in response to /blog if the said file happens to exist.

Inspired by --history-api-fallback in webpack-dev-server.

[alallier]

@alex996 thanks for the PR. Will get a review on this soon for you

[alex996]

@alallier Sounds good. I can also resolve #166 while I'm at it. It shouldn't be more than a simple fs.existsSync check. Awaiting your feedback until then.

[alallier]

@alex996 that sounds good let's do that. The rest of the PR looks good

[alex996]

@alallier Done. Both issues would be closed by this PR.

[alallier]

@alex996 It seems you wrapped the changes for #166 in the shouldFallback case. It should do that regardless of the new option,

[alex996]

@alallier Ok, I see your point. Amended the commit.

[alallier]

Looks good, merging. Thank you @alex996

[AlexanderSopov]

Hi! Awesome fix and thank you. How can I update to this version?

Best regards <3

[alallier]

@AlexanderSopov it is published to npm, you can just follow the normal npm update procedures