Added interpolation syntax for format directives.

New funtion read-format-directive and configuration variable *interpolate-format-directives*
Export *interpolate-format-directives*
Added :modify-*readtable* argument to %enable-interpol-syntax
Document the inline format directive syntax and the new argument to enable-interpol-syntax.
Deal with the case where an inline format directive takes no arguments (~~ being the canonical example)
Deal with #\v and #\V as parameter values in format directives.
Added tests for new inline format directives
Merge pull request #2 from segv/master

inline format directive syntax and an extra argument to enable-interpol-syntax