sanitize filenames before SSRing

[Rich-Harris]

fixes #1005

[codecov-io]

Codecov Report

Merging #1006 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1006   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         115      115           
  Lines        4297     4297           
  Branches     1369     1369           
=======================================
  Hits         3956     3956           
  Misses        148      148           
  Partials      193      193

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[Conduitry]

Ha I was just about to ask you to use a different character so as to not break cloning on Windows and I saw you bumped into that via CI

[Rich-Harris]

yep! not the first or last time Appveyor has saved us trouble. Background: I'm looking into the possibility of next.js-style apps (https://github.com/Rich-Harris/sapper) for Svelte, and I wanted to be able to encode parameters in file names. So far my preference is for routes/post/%id%.html -> /post/:id.