Add homepage setting

It's now possible to set the homepage to an arbitrary URL.
Take index file into account when computing parent URL

This fixes not being able to go to the parent of a directory with an
index file.
Move names of index files in util package

It will allow reusing them in multiple places.
Handle homepage like any other page
Increment version to v0.6.1
Add MIME type for .txt

This mapping doesn't seem to exist in the Kindle 3 MIME type database.
Increment version to v0.6.0
Add link to the site root
Filter escape characters from text

Didn't add an option to display them as suggested in
because they don't affect the Kindle's browser anyway.
Rename function parameter
Rename ansi.go to filter.go

In preparation for other kinds of filters.
Fix finger query parsing

Only decode space and slash.
Fix link lines with trailing spaces appearing without descriptions

While the Gemini specification v0.16.1 allows link lines without
descriptions they are impossible to follow in Kindleto. Took the
opportunity to restructure link line parsing to a separate function.
Return error for invalid client certificate scope
Serve the settings page before restarting

This prevents the "Your Kindle is unable to load the requested page."
message on the kindle when restarting.
Manage certificates from settings page
Allow serving local files