Support for floats without a leading zero

[nagy]

The file default.nix :

{
  hello = .9;
}

gives:

$ nixfmt default.nix
default.nix:2:13:
  |
2 |   hello = .9;
  |             ^
unexpected ';'
expecting expression

An even simpler testcase is: nixfmt <(echo '.1')

nixfmt may want to prepend a leading zero in this case for consistency.