Add support for the Gpmfile.

The Gpmfile serves as a dependency manager file (comparable to Gemfile in ruby).

Description of the issue:
Compile failure related to lib_ssl.cr when building src/gpm/main.cr

Expected behavior:
make install completes

Steps to reproduce:
sudo make install

Output of crystal env:
CRYSTAL_CACHE_DIR="/home/forgen/.cache/crystal"
CRYSTAL_PATH="/usr/bin/../share/crystal/src:lib"
CRYSTAL_VERSION="0.26.1"

OS Information:
Ubuntu 18.04.1 LTS
Linux aspire-e5-575 4.15.0-34-generic #37-Ubuntu SMP Mon Aug 27 15:21:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Additional Information:
/usr/bin/shards
Fetching https://github.com/mrrooijen/commander.git
Using commander (0.3.3)
/usr/bin/crystal build --release -o bin/gpm src/gpm/main.cr
_main.o: In function __crystal_main': /usr/share/crystal/src/openssl/lib_ssl.cr:213: undefined reference to SSL_library_init'
/usr/share/crystal/src/openssl/lib_ssl.cr:213: undefined reference to SSL_load_error_strings' /usr/share/crystal/src/openssl/lib_ssl.cr:213: undefined reference to OPENSSL_add_all_algorithms_noconf'
/usr/share/crystal/src/openssl/lib_ssl.cr:213: undefined reference to ERR_load_crypto_strings' _main.o: In function default_method':
/usr/share/crystal/src/openssl/ssl/context.cr:6: undefined reference to SSLv23_method' _main.o: In function matches_subject_alternative_name':
/usr/share/crystal/src/openssl/ssl/hostname_validation.cr:34: undefined reference to sk_num' /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:35: undefined reference to sk_value'
_main.o: In function ->': /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:(.text+0x5e986): undefined reference to sk_pop_free'
_main.o: In function matches_subject_alternative_name': /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:66: undefined reference to sk_pop_free'
_main.o: In function validate_hostname': /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:(.text+0x5ea77): undefined reference to sk_pop_free'
_main.o: In function matches_subject_alternative_name': /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:66: undefined reference to sk_pop_free'
_main.o: In function ->': /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:67: undefined reference to sk_free'
/usr/share/crystal/src/openssl/ssl/hostname_validation.cr:67: undefined reference to sk_free' /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:67: undefined reference to sk_free'
/usr/share/crystal/src/openssl/ssl/hostname_validation.cr:67: undefined reference to sk_free' /usr/share/crystal/src/openssl/ssl/hostname_validation.cr:67: undefined reference to sk_free'
_main.o:/usr/share/crystal/src/openssl/ssl/hostname_validation.cr:67: more undefined references to sk_free' follow collect2: error: ld returned 1 exit status Error: execution of command failed with code: 1: cc "${@}" -o '/home/forgen/temp/gpm/bin/gpm' -rdynamic -lz command -v pkg-config > /dev/null && pkg-config --libs --silence-errors libssl || printf %s '-lssl -lcrypto' command -v pkg-config > /dev/null && pkg-config --libs --silence-errors libcrypto || printf %s '-lcrypto' -lpcre -lm -lgc -lpthread /usr/share/crystal/src/ext/libcrystal.a -levent -lrt -ldl -L/usr/lib -L/usr/local/lib`
Makefile:7: recipe for target 'build' failed
make: *** [build] Error 1