Add support for user-provided templates (oapi-codegen#136)

* Add support for user-provided templates

This change introduces a new `-templates` command flag to enable users
to override code generation templates at runtime instead of having to
recompile the binary after having edited the built-in templates.

* fixup! Add support for user-provided templates

emove special characters for go names (oapi-codegen#107)

avoid index out of bounds
Fix case of ParsegetTestByNameResponse to be ParseGetTestByNameResponse
Fix &interface{}{} to be var temp interface{}\n&temp
Correctly handle Unicode

Signed-off-by: Steve Coffman <steve@khanacademy.org>

Regenerate code

Regenerate everything after pushing some template changes.

Regenerate files

I forgot to regenerate after some template white space changes.