tflint: Add workaround when parsing a config that has a trailing heredoc

[wata727]

Fixes #127
See also #93

If a plugin block ends with a heredoc (e.g. signing_key), the heredoc will not contain a trailing newline when the plugin side process parses the configuration. This does not meet the grammar of the heredoc and results in an "Unterminated template string" error.

As with #93, Unmarshal always adds a trailing newline when parsing the body. However, this is a pretty bad workaround. The biggest difference from #93 is that the added newline is meaningful. When parsing an expression, newlines are always ignored and do not affect the final parsed result, but when parsing a body the number of newlines does affect the result. For example:

plugin "aws" {
  foo = "bar"
  bar = "baz"
}

The above will be parsed as the following:

plugin "aws" {
  foo = "bar"
  bar = "baz"

}

This can have a strange effect on the display of the number of lines. However, the number of lines in a config file is only displayed when occurs a parse error mainly, so I will ignore this problem and fix the bug here.