Unexpected title format

[thibauds]

The regexp used at https://github.com/sqrthree/vuepress-theme-api/blob/99fbb2c464be321f2d2138c71f8f9412fd153808/helpers/title.js#L2 does not take accented letters into account.

Example: If you use the French word Schéma, it will result in SchéMa which is weird.

Would it be possible to let the user override this function?

Thanks for the good work by the way 👏