Add conf.d support

Fix possibly broken syntax highlighting

Add detail as to how dns-01 hook works

Respect configured BASEDIR for default values

This adds support for a new config variable CONFIG_D in the main configuration
file. Setting a path to a directoy for CONFIG_D advises letsencrypt.sh to
source any additional files found in the specified CONFIG_D directory, that
have a '.sh' ending.
By default CONFIG_D is not set, meaning letsencrypt.sh does not source any
additional configuration files.

If BASEDIR is set in config.sh, this does not modify the (default) values
of either PRIVATE_KEY, WELLKNOWN or LOCKFILE. One needs to manually specify
those three config variables in config.sh, if a different BASEDIR
should be used by letsencrypt.sh.

This commit makes letsencrypt.sh fully aware of a configured BASEDIR.

The pattern
```
 VAR="$(cmd '"')"
```
breaks syntax highlighting in some editors (e.g. gedit or jedit) due to
the tripple double quotes.

add support for  Elliptic Curve Cryptography (ECC)

check exitcode of `curl -V` because of some issue with ancient versions of curl returning error codes on checking the version number

use different sed version for different system types (mostly for compatibility with ancient linux distros nobody should be using anyway ¯\_(ツ)_/¯)

Require $WELLKNOWN only when using http-01.

Added missing space before exit code.

Check if directory for LOCKFILE is writable.

Remove home path from config.sh.example

fixed whitespace-stripping, replace all whitespace in domains.txt entries with single space character (fixes #60 and #78)

add support for dns-01 challenge - try 2