Implement GetArmoredWithCustomHeaders (#48)

* Implement GetArmoredWithCustomHeaders

ArmorWithTypeAndCustomHeaders can be reused by other PGP armoured
objects.

* Update linting, and lint accordingly

`godot` has been improved and `goerr113` has been added (and ignored
here).

* Add custom headers for keys

* Minor comment changes

Co-authored-by: Aron Wussler <aron@wussler.it>

CHANGELOG.md

@@ -4,6 +4,24 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Added
+- Key Armoring with custom headers
+```go
+(key *Key) ArmorWithCustomHeaders(comment, version string) (string, error)
+(key *Key) GetArmoredPublicKeyWithCustomHeaders(comment, version string) (string, error)
+```
+- Message armoring with custom headers
+```go
+(msg *PGPMessage) GetArmoredWithCustomHeaders(comment, version string) (string, error)
+```
+
+### Changed
+- Improved key and message armoring testing
+
+### Fixed
+- Public key armoring headers
+
 ## [2.0.1] - 2020-05-01
 ### Security
 - Updated underlying crypto library