Support token helper

[JasonCust]

In order to support Vault's token helper on reads, I came up with this little hack but it would be nice to fully support the helper within hvac (on logins, reads, etc).

def getToken():
    '''Gets the token from a Vault token helper file.

    Since the hvac package doesn't yet respect token helpers, we can
    do it outselves.
    '''

    tokenHelperFilePath = os.path.expanduser('~/.vault')

    if os.path.exists(tokenHelperFilePath):
      with open(tokenHelperFilePath, 'r') as fIn:
        tokenHelperPath = fIn.read().split('=')[1].strip().replace('"', '')
        
        if os.path.exists(tokenHelperPath):
          process = subprocess.run(
            [tokenHelperPath, 'get'], 
            stdout = subprocess.PIPE, 
            universal_newlines = True
          )

          return process.stdout


client = hvac.Client(token = getToken())

Any thoughts on if this is something that can be internally supported?