Placeables are not parsed correctly at the beginning and the end of line

[stasm]

I noticed two issues related to placeables at the beginning and the end of line:

1. In a multiline message, a placeable at the beginning of the line results in an error, while it should be allowed.

2. In a multiline message, a placeable at the end of the line consumes the \n after it.

I created fixtures for tests: stasm@f195581

(The branch is at https://github.com/stasm/fluent.js/tree/fixtures-placeable-line-extremes.)

[stasm]

@zbraniecki Would you be able to look into this today or tomorrow? Both cases are frequently used in the docs and in the default playground file. Thanks!