I'm a little hesitant to do this because most tools I'm familiar with
have the config file directly in `~/`. It's also easier to describe
where to put the file if it doesn't vary across platforms. But we're
still early in the project, so let's try it and see if we get any
complaints.
I thought that `std::fs::canonicalize()` expanded "~", but it doesn't
seem to do that, which caused #131. Git seems to do the expansion
itself, so we probably also should. More importantly
`std::fs::canonicalize()` crashes when the file doesn't exist. The
manual expansion we do now does not.
Closes#131.
