Set rc_dir based on W3M_DIR environment variable.

By default, w3m puts all of its data in the `~/.w3m/` directory
(creating it as necessary). This was not configurable in any way.

This commit adds some quick reconfigurability -- when the "W3M_DIR"
environment variable is set, w3m will use that location instead. The
default location is unchanged.

Fixes #130.

doc/FAQ.html

@@ -725,7 +725,7 @@ from Tatsuya Kinoshita <tats@debian.org>
    </dt>
    <dd>
     <p>
-     It is ~/.w3m/config.
+     By default, it is ~/.w3m/config.
     </p>
 
     <p>
@@ -734,6 +734,12 @@ from Tatsuya Kinoshita <tats@debian.org>
      setting panel. Each line contains one option setting, consisting
      of an option name and its value with a space as a separator.
     </p>
+
+    <p>
+     If the W3M_DIR environment variable is set to the name of a
+     directory, w3m will store its files in that directory instead of
+     in ~/.w3m.
+    </p>
     
     <p>Without a user-specific configuration file, w3m honours
     the system wide configuration file /etc/w3m/config.

doc/MANUAL.html

@@ -91,6 +91,11 @@ insertions tagged with "mh 2016-03-29" and "mh 2016-06-11" come from the latest
    from standard input and display it. If it doesn't find a document
    there either then normally w3m will terminate.
   </p>
+  <p>
+   You can change how w3m behaves by providing it with options, either
+   via the command line or through the configuration file. The user
+   config file is located at $W3M_DIR/config (~/.w3m/config by default).
+  </p>
   <p>
    Options include:
   </p>

doc/w3m.1

@@ -266,9 +266,16 @@ $ w3m \-v
 .EE
 .\".SH Errors
 .SH ENVIRONMENT
-\fIw3m\fP recognises the environment variable WWW_HOME as defining a
-fallback target for use if it is invoked without one.
+\fIw3m\fP recognises the environment variable \fBWWW_HOME\fP as
+defining a fallback target for use if it is invoked without one.
+
+If the \fBW3M_DIR\fP environment variable is set to a directory
+name, \fIw3m\fP will store its user files there instead of
+under the ~/.w3m directory.
 .SH FILES
+The default locations of some files are listed below. These
+locations can be altered via the \fBW3M_DIR\fP environment
+variable.
 .TP
 \f(CW~/.w3m/bookmark.html\fP
 default bookmark file