Slidify preview fails when characters "Č" and "Ę" are used in YAML field

[GegznaV]

I faced 2 problems using slidify, which are related to international characters with diacritic marks. The question is related to issue #450 and I posted the full version of questions with illustrations on StackOverflow.
In short:

1. Slidify preview fails when capital letters "Č" and "Ę" are used in YAML field with leaving error:

  Error in yaml.load(gsub("^---\n+", "", txt[1])) : 
        Reader error: control characters are not allowed: #8C at 116

Using other R markdown files this issue does not appear.

2. letters "ąčęėįšųūžĄČĘĖĮŠŲŪŽ" are displayed incorrectly on the title slide and letter "š" on all others.

My question: How can I use full functionality of slidify with the listed international letters?

[GegznaV]

If pull request in #482 is accepted, my issue will be solved.