Summary of changes

• Added Node 10 support.
• We no longer compile OpenSSL. Instead, we prefer the OpenSSL shipped with Node. In electron builds, we prefer the system openssl on linux, and we get prebuilt static libs from Conan.
• Cleaned up some compiler warnings
• Our http_parser dependency is now a submodule.
• Updated some dependencies in npm to get rid of security vulnerabilities and warnings.
• Exposed transfer progress callback for pushing.
• Libssh2 is now preconfigured for both Mac OS and Windows

Merged PRs into NodeGit

• warn about node v10 incompatibility in readme #1534
• Define error codes to fix compiler warnings about unused variables #1486
• Include http_parser via submodule #1551
• Update dependencies to get rid of security vulnerabilities #1547
• add github issue template #1548
• Enable git_push_transfer_progress - Help wanted #1500
• Fixed createBranch API params #1552
• Use curl-config to find curl #1555
• Update readme #1554
• Node 10 support #1545
• Update dependencies #1519
• Run submodule updates in sequence rather than parallel #1558
• Fix Electron builds on win32 #1560
• Use static libssh2_config.h on MacOS builds #1569