Require indent before placeables

[stasm]

A follow-up to #87 based on the discussion in #162.

In #87 (PR #159) we relaxed the indentation requirement for all non-text. This included the placeable's opening brace. For instance:

fluent/test/fixtures/select_indent.ftl

Lines 15 to 18 in a1ed83f

	select-allman-inline =
	{ $selector ->
	*[key] Value
	}

fluent/test/fixtures/variants_indent.ftl

Lines 5 to 8 in a1ed83f

	-variants-allman =
	{
	*[key] Value
	}

The discussion in #162 highlighted a possible issue for preserving some text indent caused by no indentation before placables (#162 (comment)). The issue might turn out to be solvable by the design of #162.

Just to be safe, however, should we require indent before placeables in 0.7?

[Pike]

I see leading placeables to be in the same bucket as plain white-space lines and blank lines (just \n).

They're all leading to absent and/or invisible whitespace, and I expect the fix to be the same for each.

I don't think we need special pre-cautions for placeables.

[stasm]

OK, I agree, thanks @Pike. Let's close this then.