FormattingWriter: indentAfter only if last token

Add a parameter to writeToken to only apply the token’s indentAfter if
it’s the last token of its line. For issue #37.
indentAfter for SpecifierMainToken, closes #37

The main token of a SpecifierExpression (= or =>) has an indentAfter
that’s only effective if there’s a line break immediately after the
token.