Add an Option for excludes/ignores

[ejucovy]

The RepoSearchPlugin has an Option for configuring paths to exclude from searches. Currently MultiRepoSearchPlugin does not have a comparable option.

It should! This is essential for my needs, because I have a large number of repos with mostly-identical, very-boring, and extremely crowded /static/ subdirectories. These dramatically slow down reindexing (and clutter the database) unnecessarily.

The option should probably take a comma-separated list of shell-style wildcards (​http://docs.python.org/library/fnmatch.html#module-fnmatch) or regexen to ignore.

(Ideally the implementation would be smart enough to skip subdirectories wholesale if possible rather than matching each file against the "ignores" list -- so if I have an ignore of "static/*" it should just bail out when it reaches "static/" rather than recursively walking the static/ subtree. This seems hard though since it would have to parse the ignore globs and specialcase some patterns -- it would not be allowed to bail out if the ignore is "static/*.txt" for example.)