Add a flowchart environnement and its listof

So we can more easily access those flowcharts from pdf and website,
rather than looking in the crowded list of figures.

book/sourdough-starter/sourdough-starter.tex

@@ -131,14 +131,14 @@ not airtight. I~like to use a glass and place another
 inverted one on top. The container shouldn't be airtight,
 you still want some gas exchange to be possible.
 
-\begin{figure}[!htb]
+\begin{flowchart}[!htb]
 \begin{center}
   \input{figures/fig-starter-process.tex}
   \caption[Process for starter from scratch]{The process of making a sourdough
       starter from scratch.}%
   \label{fig:sourdough-starter-process}
 \end{center}
-\end{figure}
+\end{flowchart}
 
 Now an epic battle begins. In one study scientists
 have identified more than 150 different yeast species living
@@ -256,7 +256,7 @@ your sourdough starter will catch up. Similarly, modern baking
 yeast has been isolated like this from century old sourdough
 starters.
 
-\begin{figure}[!htb]
+\begin{flowchart}[!htb]
 \begin{center}
   \input{figures/fig-starter-readiness.tex}
   \caption[Sourdough readiness flow chart]{A flow chart showing you how to
@@ -265,7 +265,7 @@ starters.
       Both are important indicators to check for readiness.}%
   \label{fig:sourdough-starter-readiness}
 \end{center}
-\end{figure}
+\end{flowchart}
 
 The key signs to look at are bubbles that you see in your starter
 jar. This is a sign that the yeast is metabolizing your
@@ -326,7 +326,7 @@ yeast part of your sourdough and balance the fermentation.
 
 \section{Maintenance}
 
-\begin{figure}[!htb]
+\begin{flowchart}[!htb]
 \begin{center}
   \input{figures/fig-starter-maintenance.tex}
   \caption[Sourdough starter maintenance flowchart]{A full flowchart showing
@@ -338,7 +338,7 @@ yeast part of your sourdough and balance the fermentation.
       accordingly when you use a stiff starter.}%
   \label{fig:sourdough-maintenance-process}
 \end{center}
-\end{figure}
+\end{flowchart}
 
 You have made your sourdough starter and your first bread. How do you perform
 maintenance for your starter? There are countless different maintenance