clean up the summary text a little

Summary: Looking at them now, some of the help summaries are confusing.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D142
support user-defined plugins

Summary:
Add search directories for plugins. On Linux/Mac we use:

1. `$XDG_CONFIG_HOME/peru/plugins`
2. `/usr/local/lib/peru/plugins`
3. `/usr/lib/peru/plugins`

And on Windows it's:

1. `%LOCALAPPDATA%\peru\plugins`
2. `%PROGRAMFILES%\peru\plugins`

Get rid of the old rule that says you can't override existing plugins --
that was from back when we allowed individual projects to provide their
own plugins, which we no longer do.

Test Plan:
We had some old testing logic for custom plugin paths, even though that
feature had been mostly removed. Repurpose that test to load plugins
from a fake home directory. Manually test on Windows.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D141
handle quotes in Content-Disposition filenames

Summary:
The HTTP standard specifies
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html#sec19.5.1) that
`Content-Disposition` filenames can be quoted. I forgot to handle that.

Closes https://github.com/buildinspace/peru/issues/70.

Test Plan: Add a couple extra cases to the existing test.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D144
reimplement shared bash scripts in python

Summary:
We have peru.sh and peru.bat, and also test.sh and test.bat. Rather than
maintaining both of those duplicates to support Windows, just
reimplement them in Python.

There's also validate-third-party.sh, but we never really use that on
Windows. We can port it in a later diff if we feel like it.

Test Plan:
Check that tests actually fail after introducing a bug. (To make sure
we're not actually testing the installed version instead of the repo
version.) Check that the linter fails after dirtying up some code. Check
that the untracked file test fails by creating a random file during
tests.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D143