Accelerate `rlx_prv_discover` provider

Introduce option `enable_shallow_app_discovery` which is disabled by
default.

When enabled searching for `*.app` files is done by using
`filelib:wildcard/1` and doesn't recursively traverse all directories
under `libs_dir` root dirs.

Instead its assumed that each directory on `libs_dir` list contains a
flat list of application dirs or is an app itself.
Added a command line override for the sys_config setting.
load scripts from files and templates
add templates
add files
set doc_root on dtl templates
add rebar_vsn_plugin directory to rebar.config
just compile regular files as template modules
fix typo in rel_name tag
assume templates are already compiled, use dtl for files too
unescape quotes
try adding some verbosity to erlydtl compilation errors
exclude dtl scripts from cover
honor existing LD_LIBRARY_PATH in script templates
ignore compiled templates
Merge pull request #128 from nuex/dtl_scripts

Make scripts in rlx_prv_assembler DTL templates
use vm.args in basic bin script
Merge pull request #137 from tsloughter/vm_args

use vm.args in basic bin script
Merge pull request #133 from mattrudder/sys_config_cmdargs

Added a command line override for the sys_config setting.
Merge pull request #101 from konradkaplita/accelerate_discover_provider

Accelerate application discovery