docs: add issue and enhancement templates (#161)

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+<!-- delete unnecessary items -->
+### Describe the bug
+
+### Steps to reproduce
+1.
+2.
+3.
+
+### Expected behaviour
+
+### Code example
+
+### Error log
+
+### Query log
+
+### Configuration
+#### Environment
+* Client version:
+* OS:
+
+#### ClickHouse server
+* ClickHouse Server version:
+* ClickHouse Server non-default settings, if any:
+* `CREATE TABLE` statements for tables involved:
+* Sample data for all these tables, use [clickhouse-obfuscator](https://github.com/ClickHouse/ClickHouse/blob/master/programs/obfuscator/Obfuscator.cpp#L42-L80) if necessary

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for the client
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+<!-- delete unnecessary items -->
+### Use case
+
+### Describe the solution you'd like
+
+### Describe the alternatives you've considered
+
+### Additional context