support **/*.md even when not on zsh

[campoy]

currently failing some tests on travis CI because embedmd **/*.md doesn't match any files

I was able to reproduce the problem by running the program from bash:

bash-3.2$ ls *.md
CONTRIBUTING.md README.md
bash-3.2$ embedmd -d **/*.md
**/*.md:open **/*.md: no such file or directory

[campoy]

decided to fix the problem by invoking it better:

find . -name "*.md" | xargs embedmd -d

might reconsider if anyone else asks for thsi