loader: replace trim_docstring with builtin

The `trim_docstring` function can be replaced by a builtin function
from the `inspect` module: `cleandoc`.

See also: https://docs.python.org/2/library/inspect.html#inspect.cleandoc

This:

```
inspect.cleandoc(func.__doc__ or '').splitlines()
```

Is functionnaly similar to this:

```
trim_docstring(func.__doc__)
```

It's even better, since the original function used to keep initial
indent and empty multi-line between non-empty lines. Since it was used
in one place only, it is safe to assume it can be replaced without
loss for the project by the builtin `inspect.cleandoc` function.

Example:

```
>>> data = """First line is not empty
...     Second line is direct after it.
...
...     Last line.
...
...     """
>>> inspect.cleandoc(data).splitlines()
['First line is not empty', 'Second line is direct after it.', '', 'Last line.']
>>> trim_docstring(data)
['First line is not empty', '    Second line is direct after it.', '', '    Last line.']
```

sopel/loader.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals, absolute_import, print_function, division
 
 import imp
+import inspect
 import os.path
 import re
 import sys
@@ -111,27 +112,6 @@ def enumerate_modules(config, show_all=False):
     return modules
 
 
-def trim_docstring(doc):
-    """Get the docstring as a series of lines that can be sent"""
-    if not doc:
-        return []
-    lines = doc.expandtabs().splitlines()
-    indent = sys.maxsize
-    for line in lines[1:]:
-        stripped = line.lstrip()
-        if stripped:
-            indent = min(indent, len(line) - len(stripped))
-    trimmed = [lines[0].strip()]
-    if indent < sys.maxsize:
-        for line in lines[1:]:
-            trimmed.append(line[:].rstrip())
-    while trimmed and not trimmed[-1]:
-        trimmed.pop()
-    while trimmed and not trimmed[0]:
-        trimmed.pop(0)
-    return trimmed
-
-
 def clean_callable(func, config):
     """Compiles the regexes, moves commands into func.rule, fixes up docs and
     puts them in func._docs, and sets defaults"""
@@ -140,7 +120,7 @@ def clean_callable(func, config):
     prefix = config.core.prefix
     help_prefix = config.core.help_prefix
     func._docs = {}
-    doc = trim_docstring(func.__doc__)
+    doc = inspect.cleandoc(func.__doc__ or '').splitlines()
     example = None
 
     func.unblockable = getattr(func, 'unblockable', False)