Add issue template

.github/issue_template.md

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+<!-- When reporting a bug in the parser, please use the following format -->
+
+* cppast version: <!-- e.g. 0.0 or latest one -->
+* parser: `libclang_parser` <!-- Change if there are multiple parsers one day -->
+* clang version: <!-- e.g. 4.0 -->
+
+Explanation of the error.
+
+Input:
+
+```cpp
+Minimal source code reproducing the error
+```
+
+Input flags: <!-- list of command line flags to compile the input, like `-std=c++14 -DFOO=BAR` -->
+
+Output:
+
+<!-- can be left out, if error not visible in cppast output -->
+
+```
+Output of `cppast -v <input>`
+```