Prefix every line of a multiline comment with #

[gaoflow]

Fixes #449.

comment("line one\nline two") only prepended # to the whole string, so every line after the first was emitted without a # and the document no longer re-parsed:

# line one
line two        # <- invalid

comment() now prefixes each line individually (a bare # for empty lines), so a multiline comment renders as a valid block of comment lines and round-trips. Single-line comments are unchanged.

[frostming]

We hope that comments will always be single-line, and multi-line comments should be rejected, otherwise it will cause potential problems, such as:

a = 1

doc["a"].comment("line1\nline2")

will produce:

a = 1 # line1
#line2

The second line should not belong to the table item any more.

So, the right approach is:

doc["a"].comment("line1")
doc.append(comment("line2"))

Losing some flexibility, the code can more clearly reflect the actual structure.