Evaluate variables in helpheader

[hgdagon]

Allow for variable usage in helpheader.

Example:

 3) Additional arguments specific to $label :
  --installdir          Set the installation directory
  --someflag            Set someflag
  --otherflag           Set some other flag

  Example:
    $0 -- --installdir=~/Applications/MyApp --someflag

Note the double empty lines at the start of the file.

[megastep]

Looking at your commit it seems like you're just moving the $helpheader from the beginning to the end though?

[hgdagon]

@megastep Yes, but the main change is on line 26, evaluating the variables in the text file.

[megastep]

Right, I didn't notice the change in the type of quotes at first. Looks good.