Add an escape sequence to avoid polymer to bind the content of brackets

[theboolean]

Refs googlearchive/TemplateBinding#186

This PR is a proposal to add the possibility to escape the brackets without binding the content.
If this kind of behaviour is acceptable, I would be glad to write also unit tests.

Usage:

{{test}}: normal behaviour
{{\test}}: don't bind to test and print literal {{test}}
{{\\test}}: don't bind to test and print literal {{\test}}
and so on... always remove one of the backslashes

[[test]]: normal behaviour
[[\test]]: don't bind to test and print literal [[test]]
[[\\test]]: don't bind to test and print literal [[\test]]
and so on... always remove one of the backslashes

[dfreedm]

This seems reasonable to me. Can you add a test case?

[theboolean]

@azakus sorry for the huuuge delay, I finally wrote tests you asked. I also rebased the history on the current master branch, so the feature is available for Polymer 2.0 and the PR can be automatically merged.

[TimvdLippe]

After a discussion about this PR, we decided to not include this functionality, mainly because of performance concerns. You can override _parseBindings in your element to implement escaping in the spots you want this feature, which means that regular applications or other elements do not regress.

[warpech]

Simple workaround that works in HTML without any overwriting of _parseBindings:

Whenever you want to write {{something}} on your page, and force Polymer to skip parsing that, put a HTML comment somewhere inside of it, e.g.

{<!-- This comment is needed to prevent Polymer from parsing curly braces -->{something}}

Tested with Polymer 2.