Templating variables and casing options

[LandonTClipp]

Description

This PR adds a few additional config options to the templating engine. The two variables added, along with their defaults, are:

structname: "Mock{{.InterfaceName}}"
outpkg: "{{.PackageName}}"

We also add more variables to the template namespace.

name	description
InterfaceNameCamel	Converts a string interface_name to InterfaceName
InterfaceNameLowerCamel	Converts InterfaceName to interfaceName
InterfaceNameSnake	Converts InterfaceName to interface_name

We delete the MockName template variable as this is not actually needed... mock names are controlled by the structname variable, and we set a default for that.

• Fixes v2.21.6: packages config does not obey --case snake #577

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Golang used when building/testing:

• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20

How Has This Been Tested?

Unit tests added

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Patch coverage: 76.74419% and project coverage change: +0.43488 🎉

Comparison is base (b61e29c) 74.49438% compared to head (32917d4) 74.92926%.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master        #578         +/-   ##
===================================================
+ Coverage   74.49438%   74.92926%   +0.43487%     
===================================================
  Files              7           7                 
  Lines           1780        1767         -13     
===================================================
- Hits            1326        1324          -2     
+ Misses           367         358          -9     
+ Partials          87          85          -2     

Impacted Files	Coverage Δ
pkg/outputter.go	34.09091% <75.60976%> (+0.58306%)	⬆️
pkg/config/config.go	71.17117% <100.00000%> (+0.26207%)	⬆️

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