Adding repo-mappings and drop-extra-header options

Updated README.md to include the two new options
Fixed build.js to work on windows
Fixed homedir lookup for windows
Moved param names to const vars at the top and replaced all references

action.yml

@@ -4,8 +4,16 @@ inputs:
     ssh-private-key:
         description: 'Private SSH key to register in the SSH agent'
         required: true
+    repo-mappings:
+        description: 'Git Repo Mappings, order and count must match ssh-private-key'
+        required: false
     ssh-auth-sock:
         description: 'Where to place the SSH Agent auth socket'
+        required: false
+    drop-extra-header:
+        description: 'Remove the .gitconfig http.extraheader auth token added by actions/checkout@v2'
+        required: false
+
 runs:
     using: 'node12'
     main: 'dist/index.js'