* throw exception if a file cannot be opened
* Add a new function in utils.hpp: open_file_or_throw: This function
returns an opened std::ifstream or throws by calling `inja_throw`.
* Use this function in Parser::load_file which previously returned an
empty string if the file couldn't be opened.
* Use this function in Environment::load_json which previously threw
a `nlohmann::detail::parse_error` if the file couldn't be opened.
* In Parser::parse_statement: When including files through `include`,
do not attempt to (re-)parse templates from files that were already
included. Additionally, this prevents inja from attempting to load
in-memory templates by their name from disk.
* Add tests that check if an exception is thrown when attempting to
open files that do not exist.
* cmake: enable C++11
* cmake: require C++11 when depending on single_inja
* code style
