Add exists_in_path jinja helper function

The function is a wrapper around `shutil.which` that allows to check for
an executable in the OS's PATH within templates.

dotdrop/jhelpers.py

@@ -6,8 +6,14 @@ jinja2 helper methods
 """
 
 import os
+import shutil
 
 
 def exists(path):
     """return true when path exists"""
     return os.path.exists(os.path.expandvars(path))
+
+
+def exists_in_path(name, path=None):
+    """return true when executable exists in os path"""
+    return shutil.which(name, os.F_OK | os.X_OK, path) is not None

dotdrop/templategen.py

@@ -49,6 +49,7 @@ class Templategen:
         self.env.globals['header'] = self._header
         # adding helper methods
         self.env.globals['exists'] = jhelpers.exists
+        self.env.globals['exists_in_path'] = jhelpers.exists_in_path
 
     def generate(self, src):
         """render template from path"""