Allow defining extensions in svelte config

[bbuhler]

Allows overriding extensions via svelte.config.js:

module.exports = {
  extensions: ['.svelte', '.html'],
};

[codecov-io]

Codecov Report

Merging #107 (47b9f49) into master (34e54d2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #107   +/-   ##
=======================================
  Coverage   94.27%   94.27%           
=======================================
  Files          41       41           
  Lines        1484     1484           
  Branches      340      341    +1     
=======================================
  Hits         1399     1399           
  Misses         85       85           

Impacted Files	Coverage Δ
src/rollup/rollupPlugin.ts	94.28% <100.00%> (ø)

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[nickreese]

@bbuhler Thanks for this. Good implementation. Can you add it to the Docs as well? https://github.com/Elderjs/docs/blob/master/elderguide.com/elderjs.md

[bbuhler]

Hi @nickreese, thanks for merging. 🙂️

Would you mind releasing a new minor/patch version with that change?
I am using a dependency from which I import HTML files as Svelte components.

Though this implementation is enough for my use case, using HTML files for Svelte components project wide would probably require more changes in various places of the Rollup config.

After this is done, I would consider adding it to the Docs.