[NewFeature] Add Get-GitIgnoreTemplateList and Get-GitIgnoreTemplate

[PlagueHO]

The cmdlets could be used to retrieve the list of .gitignore templates and download one.
Get-GitIgnoreTemplateList would contain something like this:

function Get-GitIgnoreTemplateList {
[CmdletBinding()]
param ()
return (Invoke-WebRequest -URI 'https://api.github.com/gitignore/templates' -UseBasicParsing).Content |
ConvertFrom-JSON
}

Get-GitIgnoreTemplate would be like this:

function Get-GitIgnoreTemplateList {
[CmdletBinding()]
param (
  [String]
  $TemplateName
)
}
Invoke-WebRequest -URI "https://api.github.com/gitignore/templates/$TemplateName" -UseBasicParsing |
  Select-Object -ExpandProperty Content |
  ConvertFrom-JSON |
  Select-Object -ExpandProperty Source

I can submit a PR with the changes if this is something you'd want to include.

[felixfbecker]

Feel free to submit a PR!