Rewrite custom install guide plugin to use InstallGuide plugin as base

Rather than copying all of the InstallGuide text into the custom plugin, we
now simply subclass the plugin and splice in some additional bits with some
regexes.

This also includes a fix for the example custom lib path submitted by Michael
McClimon as PR #29.

Author: autarch

inc/MyInstallGuide.pm

@@ -5,14 +5,12 @@ use Moose;
 
 use namespace::autoclean;
 
-with 'Dist::Zilla::Role::FileGatherer', 'Dist::Zilla::Role::TextTemplate';
-
-my $content = <<'EOF';
-# Installing File-LibMagic
+extends 'Dist::Zilla::Plugin::InstallGuide';
 
+my $requires_libmagic = <<'EOF';
 Installing File-LibMagic requires that you have the *libmagic.so* library and
 the *magic.h* header file installed. Once those are installed, this module is
-installed like any other Perl distributions.
+installed like any other Perl distribution.
 
 ## Installing libmagic
 
@@ -23,70 +21,34 @@ On Debian/Ubuntu run:
 On Mac you can use homebrew (https://brew.sh/):
 
     brew install libmagic
+EOF
 
-## Installation with cpanm
-
-If you have cpanm, you only need one line:
-
-    % cpanm File::LibMagic
-
-If you are installing into a system-wide directory, you may need to pass the
-"-S" flag to cpanm, which uses sudo to install the module:
-
-    % cpanm -S File::LibMagic
-
-## Installing with the CPAN shell
-
-Alternatively, if your CPAN shell is set up, you should just be able to do:
-
-    % cpan File::LibMagic
-
-## Manual installation
-
-As a last resort, you can manually install it. Download the tarball, untar it,
-then build it:
-
-    % perl Makefile.PL
-    % make && make test
-
-Then install it:
-
-    % make install
-
-If you are installing into a system-wide directory, you may need to run:
-
-    % sudo make install
-
+my $specifying = <<'EOF';
 ## Specifying additional lib and include directories
 
 On some systems, you may need to pass additional lib and include directories
 to the Makefile.PL. You can do this with the `--lib` and `--include`
 parameters:
 
-    perl Makefile.PL --lib /usr/local/include --include /usr/local/include
+    perl Makefile.PL --lib /usr/local/lib --include /usr/local/include
 
 You can pass these parameters multiple times to specify more than one
 location.
-
-## Documentation
-
-File-LibMagic documentation is available as POD.
-You can run perldoc from a shell to read the documentation:
-
-    % perldoc File::LibMagic
 EOF
 
-sub gather_files {
+sub template {
     my $self = shift;
 
-    $self->add_file(
-        Dist::Zilla::File::FromCode->new(
-            name => 'INSTALL.md',
-            code => sub {$content},
-        )
-    );
+    my $template = $self->SUPER::template;
+    $template
+        =~ s{Installing \{\{ .+ \}\} is straightforward\.\n}{$requires_libmagic}
+        or die
+        q{Could not add the "requires libmagic" text to the install guide};
+    $template =~ s{(## Manual installation)}{$specifying\n$1}
+        or die
+        q{Could not add the "specifying libmagic location" text to the install guide};
 
-    return;
+    return $template;
 }
 
 __PACKAGE__->meta()->make_immutable();