Add functionality to forcibly link items

This commit introduces an extended configuration syntax for linking
files and directories. Currently, this syntax allows for specifying
items to be forcibly linked, overwriting existing files or directories
if necessary.

The extended configuration syntax was proposed by Travers McInerney
<travers@mcinerney.me>.

README.md

@@ -91,6 +91,8 @@ have a defined ordering.
 ### Link
 
 Link commands specify how files and directories should be symbolically linked.
+If desired, items can be specified to be forcibly linked, overwriting existing
+files if necessary.
 
 #### Format
 
@@ -99,13 +101,23 @@ locations. Source locations are specified relative to the base directory (that
 is specified when running the installer). Source directory names should contain
 a trailing "/" character.
 
+Link commands support an (optional) extended configuration. In this type of
+configuration, instead of specifying source locations directly, targets are
+mapped to extended configuration dictionaries. These dictionaries map "path" to
+the source path, and specify "force" as true if the file or directory should be
+forcibly linked.
+
 ##### Example
 
 ```json
 {
     "link": {
         "~/.vimrc": "vimrc",
-        "~/.vim": "vim/"
+        "~/.vim": "vim/",
+        "~/.zshrc": {
+            "path": "zshrc",
+            "force": true
+        }
     }
 }
 ```

dotbot/executor/linker.py

@@ -1,4 +1,4 @@
-import os
+import os, shutil
 from . import Executor
 
 class Linker(Executor):
@@ -19,7 +19,15 @@ class Linker(Executor):
     def _process_links(self, links):
         success = True
         for destination, source in links.items():
-            success &= self._link(source, destination)
+            if isinstance(source, dict):
+                # extended config
+                path = source['path']
+                force = source.get('force', False)
+                if force:
+                    success &= self._delete(destination)
+            else:
+                path = source
+            success &= self._link(path, destination)
         if success:
             self._log.info('All links have been set up')
         else:
@@ -47,6 +55,22 @@ class Linker(Executor):
         path = os.path.expanduser(path)
         return os.path.exists(path)
 
+    def _delete(self, path):
+        success = True
+        if self._exists(path) and not self._is_link(path):
+            fullpath = os.path.expanduser(path)
+            try:
+                if os.path.isdir(fullpath):
+                    shutil.rmtree(fullpath)
+                else:
+                    os.remove(fullpath)
+            except OSError:
+                self._log.warning('Failed to remove %s' % path)
+                success = False
+            else:
+                self._log.lowinfo('Removing %s' % path)
+        return success
+
     def _link(self, source, link_name):
         '''
         Links link_name to source.