Work even when 'fs' not available for require

[lpw]

Sometimes the node module 'fs' is not available, so instead of failing completely, this will help 'html-to-text' regress gracefully by changing the one dependent library function fromFile to callback with a specific failure, if called at runtime.

Also, by using a string variable as an argument to require, it helps prevent webpack's static analysis from failing if it happens to try making a bundle in an fs-less environment.

Since it seems many others were also having this problem, I'm raising the PR here instead of just creating another fork with the fs code removed.

[moQuez]

oh yes.... please merge this!

[mlegenhausen]

When is fs not available?

[lpw]

In our particular case we'd like to use html-to-text in an embedded system that has no file system (hence, trying to import fs will fail). Many of the reported problems are for use in the browser (also no fs).

One could argue the merits of doing html-to-text client-side vs server-side, but it's more arguable that html-to-text should not depend on fs as there's no real need (fromFile is just a simple convenience function that could be in the calling code or should be in a separate library instead, to avoid introducing this otherwise unneeded system dependency).

Removing fromFile and its fs dependency at this point might break compatibility since people may already be using it, so this PR just makes fs conditional with a graceful fallback.

[jbwyme]

+1, would like to use this in the browser

[mlegenhausen]

Sorry for the long wait. See #171 for the progress of this feature. I will introduce the breaking change cause the upgrade process is trivial and document in the changelog.