I plan to use these templates to implement functionality
`org-brain-add-todo` in a future commit. The summary of changes in
this commit are as follows:

1. Set up capture templates for brain.

I'm using "b" as a template prefix key for capture templates related
to org-brain. Since people may already be using this key in their own
capture templates, this functionality is turned off by default (via
`org-brain-visualize-use-capture-templates` var) and the user can
select a different prefix key (via
`org-brain-visualize-capture-prefix-key` var)

Since I was setting up capture templates, I also moved the capture
template described in the README to this code.

Finally, these templates only work in the context of
org-brain-visualize, to avoid clutter in other modes.

2. `org-brain-visualize--register-capture-templates`

This is a helper function to ensure that we set up the capture
templates only once. It's internal and invisible to the user.

Add a key-binding (`C-c t`) to directly add a TODO entry to the
current entry in org-brain. This makes it possible to capture action
items about the current entry without leaving the
org-brain-visualization window.

This change depends on the capture templates introduced in the commit
before it and adds `org-brain-visualize-add-todo` and
`org-brain-add-todo` functions to enable adding tasks under a brain
entry.

Add a key-binding (`C-c z`) to directly trigger adding notes to the
current entry in org-brain. This makes it possible to capture thoughts
about the current entry without leaving the org-brain-visualization
window.

The commit provides two functions:

- `org-brain-visualize-add-note` : This function is bound to `C-c z`
in `org-brain-visualize-mode-map` and is the expected way to use this
functionality. Adds a note to the current entry.

- `org-brain-add-note`: This is a generic function that can be called
  from within org-mode.

The * causes a new heading to be created, under the heading that we
are already creating / accessing. The intention of the capture
template is to add content to an existing entry, or create a new entry
and add content to it. Both of these are done by the fixed template
now.

Earlier, these were only enabled in the `org-brain-visualize-mode`,
but it makes sense to add these everywhere, since we mostly work
ourside the *org-brain* view.

Add capture template related information to the README file as well.