Implement table formulas for clocktables.

Implement table formulas for clocktables.

Org-mode produces summaries of clocked items in the form of Org-mode
tables.  This patch implements the possibility to define formulas that
should be applied to the clock tables, in order to do additional
analysis.

Formulas for clock tables are supported in two ways.

First, a formula can simply be added by hand to a table.  Part of this
patch makes sure that, when the previous version of the table contains
a #+TBLFM: line, it will be copied to the new version.

Secondly, the clocktable definition allows a new parameter
`:formula'.  The string value of this parameter will be installed as
the formula line for the table.  If a formula parameter is given, it
will overrule any previous formula line.

As a special case, using the symbol `%' as the formula will install a
formula that will compute the fraction of total time that was spent in
a subtree.

In both cases, the formulas will be immediately evaluated after the
table has been created.