Platform error when using cygwin
```
Traceback (most recent call last):
  File "./fips", line 8, in <module>
    fips.run(fips_path, proj_path, sys.argv)
  File "/home/user/Code/fips/mod/fips.py", line 49, in run
    verb.verbs[verb_name].run(fips_path, proj_path, verb_args)
  File "/home/user/Code/fips/verbs/diag.py", line 82, in run
    check_tools(fips_dir)
  File "/home/user/Code/fips/verbs/diag.py", line 28, in check_tools
    platform = util.get_host_platform()
  File "/home/user/Code/fips/mod/util.py", line 204, in get_host_platform
    return host_platforms[platform.system()]
KeyError: 'CYGWIN_NT-6.1' 
```

What to do in this situation? At least printing a help telling to use any of the supported platforms?

diff --git a/mod/util.py b/mod/util.py
index 85a4048..3ac0c4f 100644
--- a/mod/util.py
+++ b/mod/util.py
@@ -201,6 +201,9 @@ def get_host_platform() :
 
     :returns: platform name (osx, linux, win)
     """
+    plat = platform.system()
+    if "CYGWIN_NT" in plat:
+        return host_platforms['Linux']
     return host_platforms[platform.system()]
 
 
['mod/util.py']