Enable tilde and environment variable expansion on some items

Expand environment variables in the following configuration items:
- general.pythonfile;
- general.metadata;
- mbnames.filename;
- Repository.localfolders.
- Repository.sslcacertfile.

Make tilde and environment variable expansion in the following
configuration items:
- Repository.sslclientcert;
- Repository.sslclientkey.

GitHub pull request: https://github.com/OfflineIMAP/offlineimap/pull/113
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap.conf

@@ -23,6 +23,12 @@
 
 # NOTE2: This implies that any '%' needs to be encoded as '%%'
 
+# NOTE3: Any variables that are subject to the environment variables
+# ($NAME) and tilde (~username/~) expansions will receive tilde
+# expansion first and only after this environment variables will be
+# expanded in the resulting string.  This behaviour is intentional
+# as it coincides with typical shell expansion strategy.
+
 ##################################################
 # General definitions
 ##################################################
@@ -31,6 +37,8 @@
 
 # This specifies where offlineimap is to store its metadata.
 # This directory will be created if it does not already exist.
+#
+# Tilde and environment variable expansions will be performed.
 
 #metadata = ~/.offlineimap
 
@@ -89,6 +97,8 @@ accounts = Test
 # source file and call them from this config file.  You can find
 # an example of this in the manual.
 #
+# Tilde and environment variable expansions will be performed.
+#
 # pythonfile = ~/.offlineimap.py
 #
 
@@ -135,6 +145,9 @@ accounts = Test
 # - foldername: the name of the folder;
 # - localfolders: path to the local directory hosting all Maildir
 #   folders for the account.
+#
+# Tilde and environment variable expansions will be performed
+# for "filename" knob.
 
 enabled = no
 filename = ~/Mutt/muttrc.mailboxes
@@ -375,9 +388,15 @@ remotehost = examplehost
 ssl = yes
 
 # SSL Client certificate (optional)
+#
+# Tilde and environment variable expansions will be performed.
+
 # sslclientcert = /path/to/file.crt
 
 # SSL Client key (optional)
+#
+# Tilde and environment variable expansions will be performed.
+
 # sslclientkey = /path/to/file.key
 
 # SSL CA Cert(s) to verify the server cert against (optional).
@@ -385,6 +404,9 @@ ssl = yes
 # specified, the CA Cert(s) need to verify the Server cert AND
 # match the hostname (* wildcard allowed on the left hand side)
 # The certificate should be in PEM format.
+#
+# Tilde and environment variable expansions will be performed.
+
 # sslcacertfile = /path/to/cacertfile.crt
 
 # If you connect via SSL/TLS (ssl=true) and you have no CA certificate