Fixes #585 - Added IGNORE_FILES setting for autoreload feature

See issue https://github.com/getpelican/pelican/issues/585

At first, i thought about using regular expressions for pattern matching but it would be overkill. Instead this code using a simple syntax.

```python
#In pelicanconf.py
IGNORE_FILES = ['^.#', '~$', 'TODO']
```
Above code tells pelican to avoid files that start with  .# (emacs lock files), end with ~ (emacs backup files) and files named TODO.