fluent-plugin-generate: add .gitignore file (#5370)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 
When generating a library template with `bundle gem`, a `.gitignore`
file is included by default. However, `fluent-plugin-generate` does not
currently include it, which forces developers to add it manually.

This commit adds a default `.gitignore` to the plugin template.

**Docs Changes**:
N/A

**Release Note**: 
N/A

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

templates/new_gem/.gitignore

@@ -0,0 +1,8 @@
+/.bundle/
+/.yardoc
+/_yardoc/
+/coverage/
+/doc/
+/pkg/
+/spec/reports/
+/tmp/