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

these two changes (which are really unrelated) solve two of my personal gripes with the interpol syntax:

1. I almost always end up calling `cl-interpol::%enable-interpol-syntax` directly since i want to globally modify the readtable instead of having to call `cl-interpol:enable-interpol-syntax` in every file.

So now, in my global startup function, i just call:

  (cl-interpol:enable-interpol-syntax :modify-*readtable* t)

2. I found myself wanting format like control on how the lisp values are converted to text. so with this change i can write: `#?" ~S(value) "` instead of `#?" $((prin1-to-string value)) "`, and `#?" ~X(an-integer) "` (instead of writing this i'd just fall back to format). 

Both of these changes have to be enabled by the user (the format directive interpolation must be enabled by setting `*interpolate-format-directives*` and the default value of the `modify-*readtable*` parameter is nil. I believe these are 100% backwards compatible.