Add easy way to override key algo.

Fixes csmith/docker-letsencrypt-lexicon#2

README.md

 This is passed on to Let's Encrypt, and may be used for important service
 announcements.
 
+By default this container uses Eliptic Curve keys. You can override this
+behaviour by setting the `ALGORITHM` environment variable. Dehydrated
+supports the following algorithms: `rsa`, `prime256v1` and `secp384r1`.
+
 ### Running
 
 Here's a full worked example:

config

 WELLKNOWN=/letsencrypt
 BASEDIR=/letsencrypt
 CONTACT_EMAIL=$EMAIL
-KEY_ALGO=secp384r1
+KEY_ALGO=${ALGORITHM:-secp384r1}
 
 if [ -z ${STAGING+-} ]; then
   CA="https://acme-v01.api.letsencrypt.org/directory"