patch 8.0.1451: difficult to set the python home directories properly

Problem:    It is difficult to set the python home directory properly for
            Python 2.7 and 3.5 since both use $PYTHONHOME.
Solution:   Add the 'pythonhome' and 'pythonthreehome' options. (Kazuki
            Sakamoto, closes #1266)
diff --git a/src/option.c b/src/option.c
index 08fb789..ce54369 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2248,6 +2248,15 @@
 			    {(char_u *)NULL, (char_u *)0L}
 #endif
 			    SCRIPTID_INIT},
+    {"pythonthreehome", NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
+#if defined(FEAT_PYTHON3)
+			    (char_u *)&p_py3home, PV_NONE,
+			    {(char_u *)"", (char_u *)0L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)NULL, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
     {"pythondll",   NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
 #if defined(DYNAMIC_PYTHON)
 			    (char_u *)&p_pydll, PV_NONE,
@@ -2257,6 +2266,15 @@
 			    {(char_u *)NULL, (char_u *)0L}
 #endif
 			    SCRIPTID_INIT},
+    {"pythonhome",  NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
+#if defined(FEAT_PYTHON)
+			    (char_u *)&p_pyhome, PV_NONE,
+			    {(char_u *)"", (char_u *)0L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)NULL, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
     {"pyxversion", "pyx",   P_NUM|P_VI_DEF|P_SECURE,
 #if defined(FEAT_PYTHON) || defined(FEAT_PYTHON3)
 			    (char_u *)&p_pyx, PV_NONE,